Problem with multisite install, can't change themes

Ok so I’ve got my multisite config up and running. The problem is that I upload a theme and when I network-enable them it only shows up on the settings for the main site, wich is blog.mepiro.es, but not at the second site wich is clasificados.mepiro.es

I can’t change it, I tried to enable just for one site and then to network-enable the template but it doesn’t show on the second site.

I am missing something here ? I have seen many people rewriting the .htaccess file for some reason related to not being able to change the themes in multisite, but I’m not sure this is the same problem.

EDIT: I have found another problem that could be related to the initial problem : When I click a post or some page on any of the blogs I’m being redirected on the wrong directory path and I get this error :

Not Found

The requested URL /sample-page/ was not found on this server.
Do you know where is the problem here ?

Thanks