nixos.modules.config.qt5: provide the possibility of using QT_QPA_PLATFORMTHEME="kde"

[PedroHLC]

The reason why I didn't open a PR, is described in Additional context.

Describe the bug

I'm aware of four platforms for Qt's themes right now:

• Using GTK themes through qtstyleplugins + QT_STYLE_OVERRIDE;
• Using GNOME themes through qgnomeplatform + QT_STYLE_OVERRIDE;
• Using Qt themes, with the non-KDE solution qt5ct, which uses its own color schemes;
• Using Plasma-targeted Qt themes, with kde as QT_QPA_PLATFORMTHEME, as done natively in a plasma session, using the theme's colorscheme.

Steps To Reproduce

Steps to reproduce the behavior:

1. Check qt5.platformTheme and it does not provide kde;
2. No equivalent to programs.qt5ct.enable providing kde;
3. Enable services.xserver.desktopManager.plasma5.enable = true;, then add some Pasma-target themes like [breeze-gtk breeze-icons breeze-qt5], then adjust envvars export QT_QPA_PLATFORMTHEME='kde' QT_PLATFORM_PLUGIN='kde' QT_PLATFORMTHEME='kde', set the theme in ~/.config/kdeglobals or with QT_STYLE_OVERRIDE, you'll get the right theme with the proper color scheme.

Expected behavior

qt5.platformTheme should provide kde (or plasma) as an option, it should add only the required packages to the system, and set the proper envvars. (or maybe rename it, since qt6 also uses the same variables and envvars)

Screenshots

• qbittorrent + breeze + QT_QPA_PLATFORMTHEME=kde + dark:
  

• qbittorrent + breeze + QT_QPA_PLATFORMTHEME=qt5ct + dark:
  

• qbittorrent + QT_QPA_PLATFORMTHEME=gtk2 (ignored my theme):
  

• qbittorrent + QT_QPA_PLATFORMTHEME=gnome (also ignored my theme):
  

Additional context

I tried to discover what package that is being added when evaluating services.xserver.desktopManager.plasma5 provides this platform-theme, but I failed miserably.

Notify maintainers

• stated as maintainers of plasma-integration: @ttuegel @nyanloutre
• stated as maintainer of qt5ct: @Ralith
• qt5 module contributors: @worldofpeace
• cc @thiagokokada

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

╭─pedrohlc at desktop in /home/pedrohlc
╰─λ nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 5.15.51, NixOS, 22.11 (Raccoon), 22.11.20220702.660ac43`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.9.1`
 - channels(root): `"nixos"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

[thiagokokada]

Not exactly your bug, but we should remove programs.qt5ct and integrate it with qt5 configs instead.

[romildo]

Not exactly your bug, but we should remove programs.qt5ct and integrate it with qt5 configs instead.

lxqt is another QT platform theme that can be integrated into qt5 config.