Add custom font support in theming

Is your feature request related to a problem? Please describe.
While deploying NextCloud instances, one of the things I see limiting is the current theming. There are barely any option and the possibility to follow design guides while integrating it into an environment is hard.

So far, the lack of possibility to add custom fonts is the most annoying one, which usually are a hard requirement for the environment I deploy it in.

Describe the solution you'd like
I would like to be able to upload custom fonts for their use in NextCloud theme. Similar to the OpenDyslexic toggle but with options custom CSS in case I want to upload different weights or styles for the same font.

Describe alternatives you've considered
Uploading fonts manually to the theme app, crossing the fingers they don't get an override on app update, and using the custom CSS app to add my custom fontfaces overriding default ones.

Additional context
None by now.

The issue names no files, tests, or entry points. Start by locating the existing theming implementation and the OpenDyslexic toggle, then compare how theme assets and custom CSS are handled. Done means custom fonts can be uploaded for a theme, including different weights or styles, without being lost on app updates.