Hi I'm back.
First of all I removed gnome and installed xfce instead, working great, apart from this same problem : some themes won't show up.
I ran in a terminal, and here's what i get:
/home/amaury/.themes/AmbianceRefinedII/gtk-2.0/gtkrc:110: error: unexpected identifier `arrowstyle', expected character `}'
Here's the gtkrc file:
It's in the murrine engine section, so I guess the problem comes from here. But I have murrine engine installed. Any ideas?