In reply to Richard.

re: "But you can use any theme that you desire."

Unfortunately thats not the case in my case. I tested all themes incl. the default ones.
Mostly the layout is breaking apart, the search field is spoiled and the wiki-sidebar is not showing up.
If I use the bundled theme, then the layout is ok.

It's a pity. Else it looks very promising, rich of functions and has good user control.