Allow the user to set the default theme in settings panel

[TheZoker]

The request

While setting the default system theme via automation is not a bad thing, I think that it would be easier for new users to allow them to set the default theme via the settings panel (maybe via the "General" settings tab).
It still makes sense to support setting the theme via automation (e.g set a different theme at night and at day time) but the default way (in my opinion) should be to set it via a dropdown (with all available themes) in the configuration screen.

The alternatives

Setting the theme via automation:
https://www.home-assistant.io/integrations/frontend/#theme-automation

Additional information