Add option to force Breeze/Fusion dark theme for GNOME hosts

[Mershl]

Hi,

On a GNOME host running strawberry (from flathub) will not detect that the underlying system is using a Dark Theme. The default theme of strawberry will be Breeze or Fusion Light - with no option AFAIKT to use/force Breeze or Fusion Dark.

Feature request: Add a Dark option (of the already supported themes) to Settings -> User interface -> Appearance -> Style

[jonas]

@mershl
You need to report this to flatpack.

[Mershl]

@jonas Hi Jonas, discussed the issue on #flatpak and a fix has been delivered today. flatpak now supports QGnomePlatform which fixes the issue.