Today I've been having a problem changing the site theme/logo , whenever i try and click "change theme" the next page i get says simply "user doesn' existt"

If it makes a difference i have my computer firewall disabled "since It always seems to interfer with the login system on this site" and i have the latest version of IE7.

Please help im sick of this dark grey/blue theme I'de like to look @ something brighter =)