Feature request - allow custom themes

[dhoeben]

Describe what you are trying to accomplish and why in non technical terms
I want to be able to create a custom theme so that I can integrate it better with Home Assistant when not using Home Assistant OS / want to customize my experience.

Describe the solution you'd like
Possibility to create a /config/themes/ folder and add a custom theme file. Then via config.yaml or UI enable this via maybe:

ui:
  theme: my_theme.yaml

Describe alternatives you've considered
I know that there are now a few themes to choose from in the UI

Additional context
If I can help -- let me know! Otherwise I will probably create a PR as well :-)