AppImage version of the client does not use system theme (file picker dialogs etc)

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Probably related issue https://github.com/nextcloud/desktop/issues/3555

### Expected behaviour
AppImage version of the client should use the system theme when creating file picker dialogs etc

### Actual behaviour
It does use a fallback theme

### Steps to reproduce
1. Open AppImage
2. Configure for example an account and click on choose local folder to bring up a file picker dialog.

### Client configuration
Client version: 3.3.0-rc1

Operating system: Linux

OS language:

Qt version used by client package (Linux only, see also Settings dialog):

Client package (From Nextcloud or distro) (Linux only): AppImage

Installation path of client:

### Server configuration

Nextcloud version:

Storage backend (external storage):

### Logs

Please use Gist (https://gist.github.com/) or a similar code paster for longer
logs.

1. Client logfile:
Since 3.1: Under the "General" settings, you can click on "Create Debug Archive ..." to pick the location of where the desktop client will export the logs and the database to a zip file.
On previous releases: Via the command line: `nextcloud --logdebug --logwindow` or `nextcloud --logdebug --logfile log.txt`
(See also https://docs.nextcloud.com/desktop/3.0/troubleshooting.html#log-files)

2. Web server error log:

3. Server logfile: nextcloud log (data/nextcloud.log):