After some more testing I would like to tell about some other problems.
- When I start Nautilus with gksu I have an icon in the top-left corner where I can click and edit preferences. As user I dont have this icon and cannot edit preferences.
- nm-applet does not appear on startup (also not in process list), the WiFi connection works anyway.
- date format on the panel is not editable.
- how can one edit the number of workspaces?
- how can you edit startup applications?
- Arc-Dark theme is not well suited to run libreoffice: beauty requires sacrifices.

- before changing the theme I will create a snapshot of my home directory, because I'm afraid to break it.
- task-list with text (not the Icon task list) has fixed width; when opening too many windows they do not have enough space on the panel.
- the Icon Task List does not have the settings to show only windows from current workspace.
- I would like to have an application launcher like gmrun on Alt+F2, I have no idea how to set it up. I would also like to have a shortcut for closing the current window.
- are there any plans to enable vertical panel again?