diff --git a/src/components/gui/gui.jsx b/src/components/gui/gui.jsx
index a44604898..53330d3b7 100644
--- a/src/components/gui/gui.jsx
+++ b/src/components/gui/gui.jsx
@@ -117,6 +117,7 @@ const GUIComponent = props => {
         stageSizeMode,
         targetIsStage,
         telemetryModalVisible,
+        theme,
         tipsLibraryVisible,
         vm,
         ...componentProps
@@ -303,10 +304,12 @@ const GUIComponent = props => {
                                             canUseCloud={canUseCloud}
                                             grow={1}
                                             isVisible={blocksTabVisible}
+                                            key={theme}
                                             options={{
                                                 media: `${basePath}static/blocks-media/`
                                             }}
                                             stageSize={stageSize}
+                                            theme={theme}
                                             vm={vm}
                                         />
                                     </Box>
@@ -422,6 +425,7 @@ GUIComponent.propTypes = {
     stageSizeMode: PropTypes.oneOf(Object.keys(STAGE_SIZE_MODES)),
     targetIsStage: PropTypes.bool,
     telemetryModalVisible: PropTypes.bool,
+    theme: PropTypes.string,
     tipsLibraryVisible: PropTypes.bool,
     vm: PropTypes.instanceOf(VM).isRequired
 };
@@ -448,7 +452,8 @@ GUIComponent.defaultProps = {
 
 const mapStateToProps = state => ({
     // This is the button's mode, as opposed to the actual current state
-    stageSizeMode: state.scratchGui.stageSize.stageSize
+    stageSizeMode: state.scratchGui.stageSize.stageSize,
+    theme: state.scratchGui.theme.theme
 });
 
 export default injectIntl(connect(
diff --git a/src/containers/blocks.jsx b/src/containers/blocks.jsx
index 5756249f7..cb998b929 100644
--- a/src/containers/blocks.jsx
+++ b/src/containers/blocks.jsx
@@ -18,6 +18,7 @@ import {BLOCKS_DEFAULT_SCALE, STAGE_DISPLAY_SIZES} from '../lib/layout-constants
 import DropAreaHOC from '../lib/drop-area-hoc.jsx';
 import DragConstants from '../lib/drag-constants';
 import defineDynamicBlock from '../lib/define-dynamic-block';
+import {getColorsForTheme, themes} from '../lib/themes';
 
 import {connect} from 'react-redux';
 import {updateToolbox} from '../reducers/toolbox';
@@ -94,7 +95,7 @@ class Blocks extends React.Component {
         const workspaceConfig = defaultsDeep({},
             Blocks.defaultOptions,
             this.props.options,
-            {rtl: this.props.isRtl, toolbox: this.props.toolboxXML}
+            {rtl: this.props.isRtl, toolbox: this.props.toolboxXML, colours: getColorsForTheme(this.props.theme)}
         );
         this.workspace = this.ScratchBlocks.inject(this.blocks, workspaceConfig);
 
@@ -351,7 +352,8 @@ class Blocks extends React.Component {
             return makeToolboxXML(false, target.isStage, target.id, dynamicBlocksXML,
                 targetCostumes[targetCostumes.length - 1].name,
                 stageCostumes[stageCostumes.length - 1].name,
-                targetSounds.length > 0 ? targetSounds[targetSounds.length - 1].name : ''
+                targetSounds.length > 0 ? targetSounds[targetSounds.length - 1].name : '',
+                getColorsForTheme(this.props.theme)
             );
         } catch {
             return null;
@@ -620,22 +622,11 @@ Blocks.propTypes = {
             wheel: PropTypes.bool,
             startScale: PropTypes.number
         }),
-        colours: PropTypes.shape({
-            workspace: PropTypes.string,
-            flyout: PropTypes.string,
-            toolbox: PropTypes.string,
-            toolboxSelected: PropTypes.string,
-            scrollbar: PropTypes.string,
-            scrollbarHover: PropTypes.string,
-            insertionMarker: PropTypes.string,
-            insertionMarkerOpacity: PropTypes.number,
-            fieldShadow: PropTypes.string,
-            dragShadowOpacity: PropTypes.number
-        }),
         comments: PropTypes.bool,
         collapse: PropTypes.bool
     }),
     stageSize: PropTypes.oneOf(Object.keys(STAGE_DISPLAY_SIZES)).isRequired,
+    theme: PropTypes.oneOf(themes),
     toolboxXML: PropTypes.string,
     updateMetrics: PropTypes.func,
     updateToolboxState: PropTypes.func,
@@ -656,18 +647,6 @@ Blocks.defaultOptions = {
         length: 2,
         colour: '#ddd'
     },
-    colours: {
-        workspace: '#F9F9F9',
-        flyout: '#F9F9F9',
-        toolbox: '#FFFFFF',
-        toolboxSelected: '#E9EEF2',
-        scrollbar: '#CECDCE',
-        scrollbarHover: '#CECDCE',
-        insertionMarker: '#000000',
-        insertionMarkerOpacity: 0.2,
-        fieldShadow: 'rgba(255, 255, 255, 0.3)',
-        dragShadowOpacity: 0.6
-    },
     comments: true,
     collapse: false,
     sounds: false
@@ -675,7 +654,8 @@ Blocks.defaultOptions = {
 
 Blocks.defaultProps = {
     isVisible: true,
-    options: Blocks.defaultOptions
+    options: Blocks.defaultOptions,
+    theme: 'standard'
 };
 
 const mapStateToProps = state => ({
diff --git a/src/lib/make-toolbox-xml.js b/src/lib/make-toolbox-xml.js
index 0b618979d..8d356e442 100644
--- a/src/lib/make-toolbox-xml.js
+++ b/src/lib/make-toolbox-xml.js
@@ -1,17 +1,19 @@
 import ScratchBlocks from 'scratch-blocks';
+import {defaultColors} from './themes';
 
 const categorySeparator = '<sep gap="36"/>';
 
 const blockSeparator = '<sep gap="36"/>'; // At default scale, about 28px
 
 /* eslint-disable no-unused-vars */
-const motion = function (isInitialSetup, isStage, targetId) {
+const motion = function (isInitialSetup, isStage, targetId, colors) {
     const stageSelected = ScratchBlocks.ScratchMsgs.translate(
         'MOTION_STAGE_SELECTED',
         'Stage selected: no motion blocks'
     );
+    // Note: the category's secondaryColour matches up with the blocks' tertiary color, both used for border color.
     return `
-    <category name="%{BKY_CATEGORY_MOTION}" id="motion" colour="#4C97FF" secondaryColour="#3373CC">
+    <category name="%{BKY_CATEGORY_MOTION}" id="motion" colour="${colors.primary}" secondaryColour="${colors.tertiary}">
         ${isStage ? `
         <label text="${stageSelected}"></label>
         ` : `
@@ -151,11 +153,12 @@ const xmlEscape = function (unsafe) {
     });
 };
 
-const looks = function (isInitialSetup, isStage, targetId, costumeName, backdropName) {
+const looks = function (isInitialSetup, isStage, targetId, costumeName, backdropName, colors) {
     const hello = ScratchBlocks.ScratchMsgs.translate('LOOKS_HELLO', 'Hello!');
     const hmm = ScratchBlocks.ScratchMsgs.translate('LOOKS_HMM', 'Hmm...');
+    // Note: the category's secondaryColour matches up with the blocks' tertiary color, both used for border color.
     return `
-    <category name="%{BKY_CATEGORY_LOOKS}" id="looks" colour="#9966FF" secondaryColour="#774DCB">
+    <category name="%{BKY_CATEGORY_LOOKS}" id="looks" colour="${colors.primary}" secondaryColour="${colors.tertiary}">
         ${isStage ? '' : `
         <block type="looks_sayforsecs">
             <value name="MESSAGE">
@@ -288,9 +291,10 @@ const looks = function (isInitialSetup, isStage, targetId, costumeName, backdrop
     `;
 };
 
-const sound = function (isInitialSetup, isStage, targetId, soundName) {
+const sound = function (isInitialSetup, isStage, targetId, soundName, colors) {
+    // Note: the category's secondaryColour matches up with the blocks' tertiary color, both used for border color.
     return `
-    <category name="%{BKY_CATEGORY_SOUND}" id="sound" colour="#D65CD6" secondaryColour="#BD42BD">
+    <category name="%{BKY_CATEGORY_SOUND}" id="sound" colour="${colors.primary}" secondaryColour="${colors.tertiary}">
         <block id="${targetId}_sound_playuntildone" type="sound_playuntildone">
             <value name="SOUND_MENU">
                 <shadow type="sound_sounds_menu">
@@ -343,9 +347,10 @@ const sound = function (isInitialSetup, isStage, targetId, soundName) {
     `;
 };
 
-const events = function (isInitialSetup, isStage) {
+const events = function (isInitialSetup, isStage, targetId, colors) {
+    // Note: the category's secondaryColour matches up with the blocks' tertiary color, both used for border color.
     return `
-    <category name="%{BKY_CATEGORY_EVENTS}" id="events" colour="#FFD500" secondaryColour="#CC9900">
+    <category name="%{BKY_CATEGORY_EVENTS}" id="events" colour="${colors.primary}" secondaryColour="${colors.tertiary}">
         <block type="event_whenflagclicked"/>
         <block type="event_whenkeypressed">
         </block>
@@ -382,9 +387,14 @@ const events = function (isInitialSetup, isStage) {
     `;
 };
 
-const control = function (isInitialSetup, isStage) {
+const control = function (isInitialSetup, isStage, targetId, colors) {
+    // Note: the category's secondaryColour matches up with the blocks' tertiary color, both used for border color.
     return `
-    <category name="%{BKY_CATEGORY_CONTROL}" id="control" colour="#FFAB19" secondaryColour="#CF8B17">
+    <category
+        name="%{BKY_CATEGORY_CONTROL}"
+        id="control"
+        colour="${colors.primary}"
+        secondaryColour="${colors.tertiary}">
         <block type="control_wait">
             <value name="DURATION">
                 <shadow type="math_positive_number">
@@ -429,10 +439,15 @@ const control = function (isInitialSetup, isStage) {
     `;
 };
 
-const sensing = function (isInitialSetup, isStage) {
+const sensing = function (isInitialSetup, isStage, targetId, colors) {
     const name = ScratchBlocks.ScratchMsgs.translate('SENSING_ASK_TEXT', 'What\'s your name?');
+    // Note: the category's secondaryColour matches up with the blocks' tertiary color, both used for border color.
     return `
-    <category name="%{BKY_CATEGORY_SENSING}" id="sensing" colour="#4CBFE6" secondaryColour="#2E8EB8">
+    <category
+        name="%{BKY_CATEGORY_SENSING}"
+        id="sensing"
+        colour="${colors.primary}"
+        secondaryColour="${colors.tertiary}">
         ${isStage ? '' : `
             <block type="sensing_touchingobject">
                 <value name="TOUCHINGOBJECTMENU">
@@ -504,12 +519,17 @@ const sensing = function (isInitialSetup, isStage) {
     `;
 };
 
-const operators = function (isInitialSetup) {
+const operators = function (isInitialSetup, isStage, targetId, colors) {
     const apple = ScratchBlocks.ScratchMsgs.translate('OPERATORS_JOIN_APPLE', 'apple');
     const banana = ScratchBlocks.ScratchMsgs.translate('OPERATORS_JOIN_BANANA', 'banana');
     const letter = ScratchBlocks.ScratchMsgs.translate('OPERATORS_LETTEROF_APPLE', 'a');
+    // Note: the category's secondaryColour matches up with the blocks' tertiary color, both used for border color.
     return `
-    <category name="%{BKY_CATEGORY_OPERATORS}" id="operators" colour="#40BF4A" secondaryColour="#389438">
+    <category
+        name="%{BKY_CATEGORY_OPERATORS}"
+        id="operators"
+        colour="${colors.primary}"
+        secondaryColour="${colors.tertiary}">
         <block type="operator_add">
             <value name="NUM1">
                 <shadow type="math_number">
@@ -691,25 +711,27 @@ const operators = function (isInitialSetup) {
     `;
 };
 
-const variables = function () {
+const variables = function (isInitialSetup, isStage, targetId, colors) {
+    // Note: the category's secondaryColour matches up with the blocks' tertiary color, both used for border color.
     return `
     <category
         name="%{BKY_CATEGORY_VARIABLES}"
         id="variables"
-        colour="#FF8C1A"
-        secondaryColour="#DB6E00"
+        colour="${colors.primary}"
+        secondaryColour="${colors.tertiary}"
         custom="VARIABLE">
     </category>
     `;
 };
 
-const myBlocks = function () {
+const myBlocks = function (isInitialSetup, isStage, targetId, colors) {
+    // Note: the category's secondaryColour matches up with the blocks' tertiary color, both used for border color.
     return `
     <category
         name="%{BKY_CATEGORY_MYBLOCKS}"
         id="myBlocks"
-        colour="#FF6680"
-        secondaryColour="#FF4D6A"
+        colour="${colors.primary}"
+        secondaryColour="${colors.tertiary}"
         custom="PROCEDURE">
     </category>
     `;
@@ -732,10 +754,11 @@ const xmlClose = '</xml>';
  * @param {?string} costumeName - The name of the default selected costume dropdown.
  * @param {?string} backdropName - The name of the default selected backdrop dropdown.
  * @param {?string} soundName -  The name of the default selected sound dropdown.
+ * @param {?object} colors - The colors for the theme.
  * @returns {string} - a ScratchBlocks-style XML document for the contents of the toolbox.
  */
 const makeToolboxXML = function (isInitialSetup, isStage = true, targetId, categoriesXML = [],
-    costumeName = '', backdropName = '', soundName = '') {
+    costumeName = '', backdropName = '', soundName = '', colors = defaultColors) {
     isStage = isInitialSetup || isStage;
     const gap = [categorySeparator];
 
@@ -753,15 +776,16 @@ const makeToolboxXML = function (isInitialSetup, isStage = true, targetId, categ
         }
         // return `undefined`
     };
-    const motionXML = moveCategory('motion') || motion(isInitialSetup, isStage, targetId);
-    const looksXML = moveCategory('looks') || looks(isInitialSetup, isStage, targetId, costumeName, backdropName);
-    const soundXML = moveCategory('sound') || sound(isInitialSetup, isStage, targetId, soundName);
-    const eventsXML = moveCategory('event') || events(isInitialSetup, isStage, targetId);
-    const controlXML = moveCategory('control') || control(isInitialSetup, isStage, targetId);
-    const sensingXML = moveCategory('sensing') || sensing(isInitialSetup, isStage, targetId);
-    const operatorsXML = moveCategory('operators') || operators(isInitialSetup, isStage, targetId);
-    const variablesXML = moveCategory('data') || variables(isInitialSetup, isStage, targetId);
-    const myBlocksXML = moveCategory('procedures') || myBlocks(isInitialSetup, isStage, targetId);
+    const motionXML = moveCategory('motion') || motion(isInitialSetup, isStage, targetId, colors.motion);
+    const looksXML = moveCategory('looks') ||
+        looks(isInitialSetup, isStage, targetId, costumeName, backdropName, colors.looks);
+    const soundXML = moveCategory('sound') || sound(isInitialSetup, isStage, targetId, soundName, colors.sounds);
+    const eventsXML = moveCategory('event') || events(isInitialSetup, isStage, targetId, colors.event);
+    const controlXML = moveCategory('control') || control(isInitialSetup, isStage, targetId, colors.control);
+    const sensingXML = moveCategory('sensing') || sensing(isInitialSetup, isStage, targetId, colors.sensing);
+    const operatorsXML = moveCategory('operators') || operators(isInitialSetup, isStage, targetId, colors.operators);
+    const variablesXML = moveCategory('data') || variables(isInitialSetup, isStage, targetId, colors.data);
+    const myBlocksXML = moveCategory('procedures') || myBlocks(isInitialSetup, isStage, targetId, colors.more);
 
     const everything = [
         xmlOpen,
diff --git a/src/lib/themes/__mocks__/dark-mode.js b/src/lib/themes/__mocks__/dark-mode.js
new file mode 100644
index 000000000..c3faf8d54
--- /dev/null
+++ b/src/lib/themes/__mocks__/dark-mode.js
@@ -0,0 +1,8 @@
+const blockColors = {
+    motion: {
+        primary: '#AAAAAA'
+    },
+    text: '#BBBBBB'
+};
+
+export default blockColors;
diff --git a/src/lib/themes/__mocks__/default-colors.js b/src/lib/themes/__mocks__/default-colors.js
new file mode 100644
index 000000000..f382f38aa
--- /dev/null
+++ b/src/lib/themes/__mocks__/default-colors.js
@@ -0,0 +1,11 @@
+const blockColors = {
+    motion: {
+        primary: '#111111',
+        secondary: '#222222',
+        tertiary: '#333333'
+    },
+    text: '#444444',
+    workspace: '#555555'
+};
+
+export default blockColors;
diff --git a/src/lib/themes/dark-mode.js b/src/lib/themes/dark-mode.js
new file mode 100644
index 000000000..d9fc37a57
--- /dev/null
+++ b/src/lib/themes/dark-mode.js
@@ -0,0 +1,66 @@
+const blockColors = {
+    motion: {
+        primary: '#0F1E33',
+        secondary: '#4C4C4C',
+        tertiary: '#4C97FF'
+    },
+    looks: {
+        primary: '#1E1433',
+        secondary: '#4C4C4C',
+        tertiary: '#9966FF'
+    },
+    sounds: {
+        primary: '#291329',
+        secondary: '#4C4C4C',
+        tertiary: '#CF63CF'
+    },
+    control: {
+        primary: '#332205',
+        secondary: '#4C4C4C',
+        tertiary: '#FFAB19'
+    },
+    event: {
+        primary: '#332600',
+        secondary: '#4C4C4C',
+        tertiary: '#FFBF00'
+    },
+    sensing: {
+        primary: '#12232A',
+        secondary: '#4C4C4C',
+        tertiary: '#5CB1D6'
+    },
+    pen: {
+        primary: '#03251C',
+        secondary: '#4C4C4C',
+        tertiary: '#0fBD8C'
+    },
+    operators: {
+        primary: '#112611',
+        secondary: '#4C4C4C',
+        tertiary: '#59C059'
+    },
+    data: {
+        primary: '#331C05',
+        secondary: '#4C4C4C',
+        tertiary: '#FF8C1A'
+    },
+    data_lists: {
+        primary: '#331405',
+        secondary: '#4C4C4C',
+        tertiary: '#FF661A'
+    },
+    more: {
+        primary: '#331419',
+        secondary: '#4C4C4C',
+        tertiary: '#FF6680'
+    },
+    text: '#E5E5E5',
+    workspace: '#121212',
+    toolboxSelected: '#4C4C4C',
+    toolboxText: '#E5E5E5',
+    toolbox: '#121212',
+    flyout: '#121212',
+    textField: '#4C4C4C'
+};
+
+export default blockColors;
diff --git a/src/lib/themes/default-colors.js b/src/lib/themes/default-colors.js
new file mode 100644
index 000000000..e9f9cde3c
--- /dev/null
+++ b/src/lib/themes/default-colors.js
@@ -0,0 +1,90 @@
+const blockColors = {
+    motion: {
+        primary: '#4C97FF',
+        secondary: '#4280D7',
+        tertiary: '#3373CC'
+    },
+    looks: {
+        primary: '#9966FF',
+        secondary: '#855CD6',
+        tertiary: '#774DCB'
+    },
+    sounds: {
+        primary: '#CF63CF',
+        secondary: '#C94FC9',
+        tertiary: '#BD42BD'
+    },
+    control: {
+        primary: '#FFAB19',
+        secondary: '#EC9C13',
+        tertiary: '#CF8B17'
+    },
+    event: {
+        primary: '#FFBF00',
+        secondary: '#E6AC00',
+        tertiary: '#CC9900'
+    },
+    sensing: {
+        primary: '#5CB1D6',
+        secondary: '#47A8D1',
+        tertiary: '#2E8EB8'
+    },
+    pen: {
+        primary: '#0fBD8C',
+        secondary: '#0DA57A',
+        tertiary: '#0B8E69'
+    },
+    operators: {
+        primary: '#59C059',
+        secondary: '#46B946',
+        tertiary: '#389438'
+    },
+    data: {
+        primary: '#FF8C1A',
+        secondary: '#FF8000',
+        tertiary: '#DB6E00'
+    },
+    // This is not a new category, but rather for differentiation
+    // between lists and scalar variables.
+    data_lists: {
+        primary: '#FF661A',
+        secondary: '#FF5500',
+        tertiary: '#E64D00'
+    },
+    more: {
+        primary: '#FF6680',
+        secondary: '#FF4D6A',
+        tertiary: '#FF3355'
+    },
+    text: '#575E75',
+    workspace: '#F9F9F9',
+    toolboxHover: '#4C97FF',
+    toolboxSelected: '#E9EEF2',
+    toolboxText: '#575E75',
+    toolbox: '#FFFFFF',
+    flyout: '#F9F9F9',
+    scrollbar: '#CECDCE',
+    scrollbarHover: '#CECDCE',
+    textField: '#FFFFFF',
+    insertionMarker: '#000000',
+    insertionMarkerOpacity: 0.2,
+    dragShadowOpacity: 0.6,
+    stackGlow: '#FFF200',
+    stackGlowSize: 4,
+    stackGlowOpacity: 1,
+    replacementGlow: '#FFFFFF',
+    replacementGlowSize: 2,
+    replacementGlowOpacity: 1,
+    colourPickerStroke: '#FFFFFF',
+    // CSS colours: support RGBA
+    fieldShadow: 'rgba(255, 255, 255, 0.3)',
+    dropDownShadow: 'rgba(0, 0, 0, .3)',
+    numPadBackground: '#547AB2',
+    numPadBorder: '#435F91',
+    numPadActiveBackground: '#435F91',
+    numPadText: 'white', // Do not use hex here, it cannot be inlined with data-uri SVG
+    valueReportBackground: '#FFFFFF',
+    valueReportBorder: '#AAAAAA'
+};
+
+export default blockColors;
diff --git a/src/lib/themes/high-contrast.js b/src/lib/themes/high-contrast.js
new file mode 100644
index 000000000..1c185556f
--- /dev/null
+++ b/src/lib/themes/high-contrast.js
@@ -0,0 +1,87 @@
+const blockColors = {
+    motion: {
+        primary: '#80B5FF',
+        secondary: '#B3D2FF',
+        tertiary: '#3373CC',
+        // this field is not used in the "classic" Scratch 3 color scheme
+        // If we add "modes" we'd have to handle what happens when this field is not defined.
+        quaternary: '#CCE1FF'
+    },
+    looks: {
+        primary: '#CCB3FF',
+        secondary: '#DDCCFF',
+        tertiary: '#774DCB',
+        quaternary: '#EEE5FF'
+    },
+    sounds: {
+        primary: '#E19DE1',
+        secondary: '#FFB3FF',
+        tertiary: '#BD42BD',
+        quaternary: '#FFCCFF'
+
+    },
+    control: {
+        primary: '#FFBE4C',
+        secondary: '#FFDA99',
+        tertiary: '#CF8B17',
+        quaternary: '#FFE3B3'
+    },
+    event: {
+        primary: '#FFD966',
+        secondary: '#FFECB3',
+        tertiary: '#CC9900',
+        quaternary: '#FFF2CC'
+    },
+    sensing: {
+        primary: '#85C4E0',
+        secondary: '#AED8EA',
+        tertiary: '#2E8EB8',
+        quaternary: '#C2E2F0'
+    },
+    pen: {
+        primary: '#13ECAF',
+        secondary: '#75F0CD',
+        tertiary: '#0B8E69',
+        quaternary: '#A3F5DE'
+    },
+    operators: {
+        primary: '#7ECE7E',
+        secondary: '#B5E3B5',
+        tertiary: '#389438',
+        quaternary: '#DAF1DA'
+    },
+    data: {
+        primary: '#FFA54C',
+        secondary: '#FFCC99',
+        tertiary: '#DB6E00',
+        quaternary: '#FFE5CC'
+    },
+    // This is not a new category, but rather for differentiation
+    // between lists and scalar variables.
+    data_lists: {
+        primary: '#FF9966',
+        secondary: '#FFCAB0', // I don't think this is used, b/c we don't have any droppable fields in list blocks
+        tertiary: '#E64D00',
+        quaternary: '#FFDDCC'
+    },
+    more: {
+        primary: '#FF99AA',
+        secondary: '#FFCCD5',
+        tertiary: '#FF3355',
+        quaternary: '#FFE5EA'
+    },
+    // For productionizing: we should look into making this field be the color of the text on the blocks
+    // e.g. move and steps in move (10) steps, and rename this property below for controlling the text inside
+    // of inputs e.g. 10
+    text: '#000000', // Text inside of inputs e.g. 90 in [point in direction (90)]
+
+    toolboxText: '#000000', // Toolbox text, color picker text (used to be #575E75)
+    // The color that the category menu label (e.g. 'motion', 'looks', etc.) changes to on hover
+    toolboxHover: '#3373CC',
+    insertionMarker: '#000000',
+    insertionMarkerOpacity: 0.2,
+    fieldShadow: 'rgba(255, 255, 255, 0.3)',
+    dragShadowOpacity: 0.6
+};
+
+export default blockColors;
diff --git a/src/lib/themes/index.js b/src/lib/themes/index.js
new file mode 100644
index 000000000..e1fa87f71
--- /dev/null
+++ b/src/lib/themes/index.js
@@ -0,0 +1,31 @@
+import defaultsDeep from 'lodash.defaultsdeep';
+
+import darkMode from './dark-mode';
+import highContrast from './high-contrast';
+import defaultColors from './default-colors';
+
+const mergeWithDefaults = colors => defaultsDeep({}, colors, defaultColors);
+
+const themeMap = {
+    'dark-mode': mergeWithDefaults(darkMode),
+    'high-contrast': mergeWithDefaults(highContrast),
+    'standard': defaultColors
+};
+
+const getColorsForTheme = theme => {
+    const colors = themeMap[theme];
+
+    if (!colors) {
+        throw new Error(`Undefined theme ${theme}`);
+    }
+
+    return colors;
+};
+
+const themes = Object.keys(themeMap);
+
+export {
+    defaultColors,
+    getColorsForTheme,
+    themes
+};
diff --git a/src/reducers/gui.js b/src/reducers/gui.js
index 378a3203c..2e70c40e1 100644
--- a/src/reducers/gui.js
+++ b/src/reducers/gui.js
@@ -21,6 +21,7 @@ import fontsLoadedReducer, {fontsLoadedInitialState} from './fonts-loaded';
 import restoreDeletionReducer, {restoreDeletionInitialState} from './restore-deletion';
 import stageSizeReducer, {stageSizeInitialState} from './stage-size';
 import targetReducer, {targetsInitialState} from './targets';
+import themeReducer, {themeInitialState} from './theme';
 import timeoutReducer, {timeoutInitialState} from './timeout';
 import toolboxReducer, {toolboxInitialState} from './toolbox';
 import vmReducer, {vmInitialState} from './vm';
@@ -55,6 +56,7 @@ const guiInitialState = {
     fontsLoaded: fontsLoadedInitialState,
     restoreDeletion: restoreDeletionInitialState,
     targets: targetsInitialState,
+    theme: themeInitialState,
     timeout: timeoutInitialState,
     toolbox: toolboxInitialState,
     vm: vmInitialState,
@@ -154,6 +156,7 @@ const guiReducer = combineReducers({
     fontsLoaded: fontsLoadedReducer,
     restoreDeletion: restoreDeletionReducer,
     targets: targetReducer,
+    theme: themeReducer,
     timeout: timeoutReducer,
     toolbox: toolboxReducer,
     vm: vmReducer,
diff --git a/src/reducers/theme.js b/src/reducers/theme.js
new file mode 100644
index 000000000..5d05eff00
--- /dev/null
+++ b/src/reducers/theme.js
@@ -0,0 +1,25 @@
+const SET_THEME = 'scratch-gui/theme/SET_THEME';
+
+const initialState = {
+    theme: 'standard'
+};
+
+const reducer = (state = initialState, action) => {
+    switch (action.type) {
+    case SET_THEME:
+        return {...state, theme: action.theme};
+    default:
+        return state;
+    }
+};
+
+const setTheme = theme => ({
+    type: SET_THEME,
+    theme
+});
+
+export {
+    reducer as default,
+    initialState as themeInitialState,
+    setTheme
+};
diff --git a/test/unit/util/themes.test.js b/test/unit/util/themes.test.js
new file mode 100644
index 000000000..d281add0c
--- /dev/null
+++ b/test/unit/util/themes.test.js
@@ -0,0 +1,22 @@
+import {defaultColors, getColorsForTheme} from '../../../src/lib/themes';
+
+jest.mock('../../../src/lib/themes/default-colors');
+jest.mock('../../../src/lib/themes/dark-mode');
+
+describe('themes', () => {
+    test('provides the default theme colors', () => {
+        expect(defaultColors.motion.primary).toEqual('#111111');
+    });
+
+    test('returns the dark mode', () => {
+        const colors = getColorsForTheme('dark-mode');
+
+        expect(colors.motion.primary).toEqual('#AAAAAA');
+    });
+
+    test('uses default theme colors when not specified', () => {
+        const colors = getColorsForTheme('dark-mode');
+
+        expect(colors.motion.secondary).toEqual('#222222');
+    });
+});