Dark theme / respecting system settings

[jancborchardt]

Currently the app store only supports light mode. It would be nice if it would respect the system settings / also be available in dark mode.

We could use the same variables as we use for the web interface.

[DaphneMuller]

Hi Jan, as we talked about it in the planning call, this is for now not a priority of the maintainers. If someone would like to do a pull request we would swiftly review the PR.