Found a way to solve this.
Not really the way but its working.
Copy the Style-sheet in the Zip to DirAssets.
Use the following code after a form is created and Initialize.
MainForm.Stylesheets.Add(File.GetUri(File.DirAssets, "caspian.css"))
I hope this helps somebody.
There can be a Warning in the Log of the IDE.
WARNING: Could not resolve 'a lookup value' while resolving lookups for '-fx-border-color' from rule '*.table-cell' in stylesheet file:/../../caspian.css
I don't know how to solve this.