EEP and JD, thank you so much for your help.
JD, I did as you suggested, and X failed to start; but then I removed the "Device" section altogether and it worked great!
I did so because I felt it was just a matter of setting the color depth, and in fact it seems that everything's working now.
I also tried to manually fix the problem i had with debian-installer and it seems there's a bug in the system of dependencies, in this sense: package "mint-artwork-gnome" depends on a "gtk2-engines-clearlooks" which really doesn't exist, because engine clearlooks is shipped together with "gtk2-engines".
However, manually installing all the requested engines did the trick.
Now, only some programs seem to expect that gtk theme is set by gnome (they don't use the one I choose, they fall back to the default theme). Among them, Abiword, gnome-mplayer and some utilities like "date and time".
Do you have any suggestions to fix their appearance to fit my theme?
Thank you again!