[Bug]: Theme Setting is broken on NixOS

[Computerdores]

Checklist

• I am using an up-to-date version.
• I have read the documentation.
• I have searched existing issues.

TagStudio Version

8347337

Operating System & Version

NixOS 26.05

Description

Dark Mode does not activate eventhough theme is set to "Dark" and TS was restarted:

Expected Behavior

When theme is set to "Dark" application should be dark 🙃

Steps to Reproduce

1. set Theme to "Dark"
2. restart TS
3. observe flashbang
4. be blind

Logs

No response

[xarvex]

Do other QT applications show up in dark mode? If you don't have one handy to try you can test out nomacs . QT theming can be difficult outside GNOME or KDE, I've had to do weird stuff in home manager.

[Computerdores]

Yeah nomacs works (if I go into the settings and enable dark mode; automatic detection doesn't work either but that is a separate issue: #287)

[xarvex]

Alright, in that case: when you launch TagStudio, is this through the devshell or is the packaged application started (either through command or desktop item)? If this is happening within the devshell, it may be because of the external (to Nix) pip packages I'd wager.

[Computerdores]

both nix run and nix develop show this behaviour

[Computerdores]

Ok I have investigated a little and using QT_LOGGING_RULES="qt.qpa.theme=true" one can see what Qt Platform Theme is used.
Normally it uses qt5ct:

qt.qpa.theme: Successfully created platform theme "qt5ct" via QPlatformThemeFactory::create

If I force the gtk3 platform theme using QT_QPA_PLATFORMTHEME=gtk3 the dark theme works again.

This means that the qt5ct dark theme seems to not work with TS for me

[Computerdores]

If I force the gtk3 platform theme using QT_QPA_PLATFORMTHEME=gtk3 the dark theme works again.

I have been testing this workaround and it turns out that this breaks the file picker (some error related to GSettings, which is a GNOME specific thing)