How to get Dark Mode to work in Qt apps in AppImages?

[tpimh]

I am trying to package a Qt application in an AppImage. The app always has the default look regardless of the OS setting to use Dark Mode.

I tried to solve this problem by manually adding libqt5ct.so and libqt5ct-style.so to the AppImage, but this didn't change anything. My qt5ct outside of AppImage is set to use "Fusion" style with "darker" color scheme, but I still get the default look when launching the AppImage.

[srilakshmikanthanp]

I am also facing the same issue have you managed to fix this?

[tpimh]

Nope, I believe this is just a limitation at the moment. I am not aware of anyone working on this.

[srilakshmikanthanp]

I have used --custom-apprun script to set environment variable QT_QPA_PLATFORMTHEME=xdgdesktopportal after that theme detection works, In my case it seems to be set as gtk2 in default apprun's hook so theme detection not worked

[TheAssassin]

Someone will have to research what files need to be added exactly before this issue can be resolved.

[srilakshmikanthanp]

For me, the issue was caused by this hook. I resolved it by adding a custom application run script to override the QT_QPA_PLATFORMTHEME environment variable, setting it to xdgdesktopportal. This made the application to detect theme properly on Ubuntu 24.

Does it make sense to have a custom hook to handle this on the user's end?

Here's the original snippet:

# generated by linuxdeploy-plugin-qt

# try to make Qt apps more "native looking" on Gtk-based desktops, if possible
# see https://github.com/AppImage/AppImageKit/issues/977#issue-462374883
case "${XDG_CURRENT_DESKTOP}" in
    *GNOME*|*gnome*|*XFCE*)
        export QT_QPA_PLATFORMTHEME=gtk2
        ;;
esac

[TheAssassin]

@srilakshmikanthanp does removing the hook altogether fix the problem, too? I'd consider removing it entirely (or at least not overwrite an existing value).

[srilakshmikanthanp]

@TheAssassin In my case, theme detection only works if I set export QT_QPA_PLATFORMTHEME=xdgdesktopportal. I verified this by unsetting it (export QT_QPA_PLATFORMTHEME=""), and theme detection stopped working.

Would it be acceptable to allow users to optionally provide a hook file so they can configure this on their end if needed, with an empty hook or current behavior as the default.

[TheAssassin]

Define users. End users of your application? Users of linuxdeploy who build app bundles?

[srilakshmikanthanp]

Users of linuxdeploy who build app bundles.

[TheAssassin]

As I said I am open to removing the hook altogether, I am not sure if it is still helpful at all. Plus, I already suggested that we can just accept a predefined value.

I am not sure that setting xdgdesktopportal is universally OK, especially on older, still-supported distros. That would have to be tested.

I am open to parameterizing this, though.

[srilakshmikanthanp]

If the default app run script only launches the application without any additional setup, users can simply replace it with a custom script to handle all required tasks, such as setting the QT_QPA_PLATFORMTHEME environment variable. In that case, removing the hook altogether would be fine. However, if the default script performs more than just starting the app, offering a way to inject a custom hook script would be valuable. It would let users run additional steps before executing app without needing to override the entire default run.

In my case, I’ve overridden the default app run script to set the QT_QPA_PLATFORMTHEME environment variable and then simply launched the application. So, removing the hook mechanism would be fine for my case.

I am not sure that setting xdgdesktopportal is universally OK, especially on older, still-supported distros. That would have to be tested.

Yes we should not set any value to that, it would be better to set by user based on their requirements. Also we can put this note in some docs so if some one has same problem they can fix it without spending much time

[scrubbbbs]

I also agree it would be better if this was not set at all, and let it be up to users running the AppImage. I used to use qt6ct on my desktop (which requires QT_QPA_PLATFORMTHEME=qt6ct) , and this hook would break that.

Also the gtk2 platform theme (libqtgtk2.so) is not typically installed in Qt6, and there is no apt package for it.

[TheAssassin]

I suppose we should get rid of the hook script for Qt 6 then at least. I'll try to add a switch later.

[TheAssassin]

The latest version no longer creates that hook file. Please test and report.