netbox/docs/plugins/development/user-interface.md

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:

window.addEventListener('netbox.colorModeChanged', e => {
  const customEvent = e as CustomEvent<ColorModeData>;
  console.log('New color mode:', customEvent.detail.netboxColorMode);
});