GTK theme does not work properly on Qt Startup Applications

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?

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?