fix(theme): Disable broken gnome theme

As QGnomePlatform is no longer maintained, the
theme should be set to KDE by default. This fixes
broken rendering on newer gnome desktops.

com.nextcloud.desktopclient.nextcloud.yml

@@ -23,6 +23,7 @@ finish-args:
   - --own-name=com.nextcloudgmbh.Nextcloud
   - --talk-name=org.kde.kwalletd5
   - --env=TMPDIR=/var/tmp
+  - --env=QT_QPA_PLATFORMTHEME=kde
 
 cleanup:
   - /share/icons/hicolor/1024x1024