Support FreeDesktop dark mode preference

[vchernin]

Guidelines

• I have read the guidelines.

Version/Commit hash

No response

Describe the bug.

Enabling dark mode with traditional themes can be a subpar experience, since there is no real agreed upon standard for how dark mode should work on Linux.

Expected behavior

This is fixed by the introduction of FreeDesktop dark mode preference. See flatpak/xdg-desktop-portal#629 and https://blog.elementary.io/the-need-for-a-freedesktop-dark-style-preference/

This should mean more reliable and consistent dark mode behaviour.
Likely needs support in the toolkit/UI library, so this is probably blocked on a decision in https://github.com/tenacityteam/tenacity/issues/572

OS

No response

Additional context

No response

This issue is not a duplicate

• I have confirmed this issue isn't a duplicate.

[n0toose]

This could work much better and in a similar manner as https://github.com/tenacityteam/tenacity/pull/608.

[n0toose]

Went on the thread to thank the very nice freedesktop people for working on that. I'll see to it.

[BrainBlasted]

Here's some example code on how to do this with SDL and GDBus, courtesy of Alexander Mikahaylenko: https://gist.github.com/BrainBlasted/931b763fc3dcdec2fc55a4ef64fca1ba

[vchernin]

See https://blogs.gnome.org/alexm/2021/10/04/dark-style-preference/

[n0toose]

Related to https://github.com/tenacityteam/tenacity/issues/686#issuecomment-1023460496