Fix dark mode detection on Linux

[benthecarman]

Enable GTK3 platform theme integration on Linux to properly detect
system dark/light mode settings. Without this, Qt applications on
Linux don't automatically adapt to the system theme, unlike macOS
where it works out of the box.

The fix sets QT_QPA_PLATFORMTHEME to "gtk3" before QApplication
initialization, but only if the user hasn't already set a platform
theme. This respects existing user configurations while providing
better defaults for desktop environments using GTK (GNOME, Unity,
XFCE, MATE).

The change is non-breaking as Qt falls back gracefully if the GTK
platform theme plugin is unavailable

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[hebasto]

Enable GTK3 platform theme integration on Linux to properly detect
system dark/light mode settings. Without this, Qt applications on
Linux don't automatically adapt to the system theme, unlike macOS
where it works out of the box.

Could you please clarify which build this change is intended to affect, statically linked or shared?

If the latter, which system was this tested on (OS, desktop environment, Qt version)?

[luke-jr]

Seems like this could make things worse for native (Qt) environments?

[benthecarman]

Could you please clarify which build this change is intended to affect, statically linked or shared?

If the latter, which system was this tested on (OS, desktop environment, Qt version)?

this sadly only effects dynamically linked builds.

I have tested on ubuntu 24.04 with Qt 6.4.2

[hebasto]

Could you please clarify which build this change is intended to affect, statically linked or shared?
If the latter, which system was this tested on (OS, desktop environment, Qt version)?

this sadly only effects dynamically linked builds.

Bringing this functionality to statically linked (including release) binaries would require introducing new build dependencies. There is a consensus among Core developers that this should only be considered after separating the GUI build into its own Guix script.

I have tested on ubuntu 24.04 with Qt 6.4.2

On the same system, the master branch @ 34fefb6:

Everything seems to be working correctly.

[katesalazar]

Ah, nice and small, thanks. I have a couple of questions.

This is in your code:

This must be done before creating the QApplication

Does this mean window won't change live when theme changes, on Linux?

This is in your post:

unlike macOS where it works out of the box.

Does this mean window will change live when theme changes, on macOS?