fix: apply selected theme also to native parts of the app

[ShGKme]

☑️ Resolves

A minor change in theming. Simplifying pages setup and respects selected theme in all parts of the app.

• Before we used Nextcloud theme via [data-theme-*]. It has 2 drawbacks:
  • Requires setting attrs initially on every window
  • Native parts of the app have a system theme
• Now using a native theme:
  • Requires settings only once on app init
  • Works on all the parts

🖼️ Screenshots

Example - a Light theme on a Dark system

🏚️ Before	🏡 After

[Antreesy]

Tested