Theme in Display Settings is applied without pressing save

[5argon]

Most settings in the https://arkham.build/settings page requires pressing the "Save settings" button to confirm, otherwise it reverts to the previous value. However, only the Theme settings stays as previewed without pressing Save.

[fspoettel]

This one is a bit tricky - we need to save the theme to local storage so we can apply it synchronously immediately. The rest of the settings is loaded async in app bootstrap. Needs a bit of extra logic to make it consistent.