Hi,
I suppose you're not looking for an answer anymore, kyriakos. But for others with the same question, here's the answer.
Each site uses the same theme as the general Alfresco dashboard per default, but this can be changed for every individual site. Go to a site, and in the right upper corner click "More" and then "Customize site". On this page you can set the theme for this specific site (under "Site Theme").
Hope this may help someone.
Kind regards,
Leon
