Closes #18006: Dispatch event when toggling color mode & document for plugin use (#20031)

docs/plugins/development/user-interface.md

@@ -0,0 +1,14 @@
+# User Interface
+
+## Light & Dark Mode
+
+The NetBox user interface supports toggling between light and dark versions of the theme. If needed, a plugin can determine the currently active color theme by inspecting `window.localStorage['netbox-color-mode']`, which will indicate either `light` or `dark`.
+
+Additionally, when the color scheme is toggled by the user, a custom event `netbox.colorModeChanged` indicating the new scheme is dispatched. A plugin can listen for this event if needed to react to the change:
+
+```typescript
+window.addEventListener('netbox.colorModeChanged', e => {
+  const customEvent = e as CustomEvent<ColorModeData>;
+  console.log('New color mode:', customEvent.detail.netboxColorMode);
+});
+```