Have you tested whether you can change to a new skin via "my
preferences"?
Say, choose Modern or Classic instead of MonoBook?
That would at least test that it is possible to choose another skin in
your
installation.
That's a very good question which didn't occur to me to ask. At the moment, I've only registered two users: myself and "Admin".
Using both accounts although the GUI allows me select a change of skin via "my preferences", the actual change doesn't take effect. So what does this tell you?