Change theme accroding to System Theme

[snpefk]

Is your feature request related to a problem? Please describe.
Foliate has different themes, and depending on the time I manually change them for better readability and eye comfort.

Describe the solution you'd like
Change theme according to a System Theme. Other apps (like Firefox or Nautilus) can do that.

Describe alternatives you've considered
Just do it manually.

Additional context
🐬

[johnfactotum]

This has long been fixed in the gtk4 branch.