Thanks for the reply. However, it seems that there is something that is not right with my installation...
I've just decided to reinstall ezPublish (on a Debian 5.0 by the way) and I chose the Website Interface (ver. 1.6-0) WITH demo content, but when the installation is complete, there is just no content (well, that's what I believe). http://img820.imageshack.us/f/ezpublish1.png/
I think I should first solve this problem. But I don't know exactly why this is happening.
Ok, so I've managed to display the demo content. Actually, I did something wrong during the installation at this screen : http://img121.imageshack.us/i/ezpublish3.png/
During the first installations, I chose "Skip" and that's why I did not get any demo content. On my last installtion, I simply chose "Map to French" and now I've got everything.
However, it seems that I still can't change the theme, even by emptying the cache of my browser.