Support FreeDesktop dark mode preference

Guidelines

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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 https://github.com/flatpak/xdg-desktop-portal/issues/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

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This issue is not a duplicate

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