Use dark flyout checkboxes in dark mode

src/lib/tw-theme-dark.css

@@ -36,6 +36,19 @@
 .blocklyFlyoutButton:hover {
     fill: #111;
 }
+/* blocklyFlyoutCheckboxPath stroke and blocklyFlyoutCheckbox fill must match */
+.blocklyFlyoutCheckboxPath {
+    stroke: #111;
+}
+.blocklyFlyoutCheckbox {
+    fill: #111;
+}
+.checked > .blocklyFlyoutCheckbox {
+    stroke: #a1c6fa;
+}
+.checked > .blocklyFlyoutCheckboxPath {
+    stroke: white;
+}
 .scratchCategoryMenu {
     color: #ccc;
 }