When I select the account option at the top of the page it just goes blank so i've found this method easier if it helps anyone.
Sounds a bit weird but the best way I have found to change anything is to select 'private messages' from the main page and then click on 'change theme' etc etc at the top.
As Dave has mentioned, although 'aphrodite' and 'kashmir' are ok, you don't get the full facilities that 'platinum' has, you loose the last 5 posts on the forum for a start which is very helpful while getting used to the forum area of the site.
pj