GTK theme does not work properly on Qt Startup Applications

Question

Setup

When a Qt app (KeePassXC, AntiMicroX) is started automatically (via autostart), the GTK theme is not applied.

All apps are installed via standard apt packages (I am aware of snap theming issues).
I am on Ubuntu 20.04, I use the Yaru dark gtk theme.
To apply the GTK theme onto Qt apps, I did the following:
- Installed the qt5-style-plugins apt package (with all of its dependencies)
- Added a line to /etc/environment: QT_QPA_PLATFORMTHEME=gtk2
- This works properly if I start the applications manually

Additional fiddling showed, that on cold start the theme never gets applied properly, however, on a restart, it does work sometimes.

Questions

How can I ensure that the dark gtk application theme gets properly used?
I would assume that when the autostart happens, some setting is not loaded, hence the breakage. Is there some sort of "service" those apps would have to wait for?
Should I try to add delay?

By any chance, are these installed as snap? Edit your question to add this information — Archisman Panigrahi, Dec 07 '20 at 11:21
@ArchismanPanigrahi No - standard apt installs. snap theming is a different problem I gave up on (some say its work in progress) — MrMeszaros, Dec 07 '20 at 11:29
Check out the package qt5ct, it's a qt5 configuration tool, that might be helpful for you. — mook765, Dec 13 '20 at 23:51
I should point out that Qt-based applications don't inherit GTK themes - they use Qt themes. Just like GTK applications don't use Qt themes. Known issue between those types of applications using different themeing mechanisms. — Thomas Ward, Dec 14 '20 at 13:59
@mook765 Thanks, you were right - in the meantime I did some local testing. However, I had no success with qt5ct - the theme still does not apply properly with autostart. Do You know more about autostart and theming please share some sources! — MrMeszaros, Dec 14 '20 at 15:11
Does it work when you add a startup delay? Also, there may be issues with qt5ct. You can try Kvantum, which is modern, and very actively developed. See https://askubuntu.com/a/1234625/124466 — Archisman Panigrahi, Dec 17 '20 at 12:03

MrMeszaros · Accepted Answer · 2020-12-21T16:22:59.530

What made it work

Added a 1 second startup delay to the autostart desktop files (in ~/.config/autostart/):

[Desktop Entry]
...
X-GNOME-Autostart-Delay=1

This made the apps to start after the X session has properly initialized itself after login. Nothing else was needed (apart from the initial setup).

Notes

The QT Configuration tool (shipped in qt5ct apt package) is viable but not necessary.
- If installed it addes an X session config /etc/X11/Xsession.d/99qt5ct that sets the QT_QPA_PLATFORMTHEME to qt5ct if unset.
- It enables a wider configuration of Qt theming. I used a minimal ~/.config/qt5ct/qt5ct.conf:
```
[Appearance]
icon_theme=Numix-Circle
standard_dialogs=gtk2
style=gtk2
```
For debugging the env of the apps can be checked with this shell command (note, I use fish and not bash): strings /proc/(pidof keepassxc)/environ | grep QT

To investigate where environment variables are set one can use these commands:

# system wide - this was quick
grep QT_QPA_PLATFORMTHEME --recursive /etc/*
# per user - this was super slow on my system (*khm* steam *khm*)
grep QT_QPA_PLATFORMTHEME --recursive ~/*

Sources:
- Comments under this question
- How to determine where an environment variable came from?
- How can I delay a specific program on startup?

score 0 · Answer 2 · answered Nov 06 '21 at 04:47

On Xubuntu 20.04 LTS

First, install the Qt5 configuration tool with

$ sudo apt install qt5ct

Then, comment out the following line in /etc/X11/Xsession.d/56xubuntu-session by inserting a hash up front:

sudo nano /etc/X11/Xsession.d/56xubuntu-session
    # QT5 apps to use GTK style
    #export QT_QPA_PLATFORMTHEME=gtk2

Now, log out and in again. Changes made with Menu → Settings → Qt5 Settings will take immediate effect.