How do I package a Qt style plugin?

[Timo Wilken]

Hi Guixers,

I'm wondering what the right way is to package Qt plugins in Guix.
Specifically, I'd like to use Lightly: https://github.com/Luwx/Lightly

While I've managed to build it (see my package definition at the end
of this email), I can't seem to get Qt applications to actually use
Lightly as a style plugin.

For example, nheko (which is packaged in (gnu packages messaging)):

$ QT_STYLE_OVERRIDE=lightly nheko 
QApplication: invalid style override 'lightly' passed, ignoring it.
        Available styles: Windows, Fusion

If I `cat $(which nheko)', it turns out to be a shell script
containing a line like the following:

export QT_PLUGIN_PATH="/gnu/store/...-qtbase-5.15.5/lib/qt5/plugins:..."

...which does not contain a path to Lightly, even though it is
installed in the same Guix profile as nheko.

What do I need to do to make installed Qt applications like nheko
"see" the Lightly Qt plugin?

Thanks,
Timo


P.S.: Here's my full package definition for Lightly:

(define-public lightly
  (package
   (name "lightly")
   (version "0.4.1")
   (source
    (origin
     (method git-fetch)
     (uri (git-reference
           (url "https://github.com/Luwx/Lightly";)
           (commit (string-append "v" version))))
     (file-name (git-file-name name version))
     (sha256 (base32 "0qkjzgjplgwczhk6959iah4ilvazpprv7yb809jy75kkp1jw8mwk"))))
   (build-system qt-build-system)  ; cmake-build-system should be fine too
   (native-inputs
    (list extra-cmake-modules qtbase-5 qtdeclarative-5 qtx11extras
          kconfigwidgets kcoreaddons kdecoration kguiaddons
          ki18n kiconthemes kwindowsystem
          kde-frameworkintegration  ; optional; Required to use KStyle 
convenience functionalities in style
          kcmutils                  ; optional; Required for lightly-settings5 
application to build
          kwayland                  ; optional
          pkg-config))              ; optional
   (home-page "https://github.com/Luwx/Lightly";)
   (synopsis "A modern style for qt applications")
   (description "Lightly is a fork of breeze theme style that aims
to be visually modern and minimalistic.")
   (license license:gpl2+)))