Cloussau,
To access the themes, makes sure your logged into the forums first, then click the profile button at the top of the page, then on the left side of the new screen click 'look and layout preferences', then it ill say something like 'Current Theme: ALWIL classic Theme (change)' near the top, click '(change)' to choose from a list of themes, then click 'apply new theme' to keep it.
--lee