fix: handle system theme change

[ShGKme]

☑️ Resolves

Preparation to complete theme change support.

• Before, there was no support for the default Nextcloud theme (= system preferences)
  • There where only light and dark theme
  • App checked system preferences only on init
• Now:
  • Also load so called plain theme styles with prefers-color-scheme like the server does
  • Initially set default theme instead of specific
  • This allows handling system theme change in a native way like server does

🖼️ Screenshots

theme-change.mp4

[ShGKme]

Note: avatar change is a new feature of spreed PR

[Antreesy]

Tested, really nice!

Don't you want to add a toggle to control it locally in client?

[ShGKme]

Don't you want to add a toggle to control it locally in client?

I do, it's several separated PRs.

Just found this issue in progress. Debugging system theme switch didn't work in Talk Desktop