I tried to install the Theme "Blue XMB" ( http://psv.altervista.org/theme.php?id=1470159415 ) and the message that it was installed successfully was shown. So I restarted my console and nothing changed. I did have this Theme before in 3.36 and would like to use it again.
I just recently updated to 3.60 and HENkaku, so my Database was rebuilt recently (during the update process).
If there is any more Info needed, let me know, I'll have an eye on this Issue.