Didn't even realize it was visible for Opera/Moz users... well, I knew about the option, just never laid the connection with testing a theme ...
/me sticks out his tongue to JohnnyFist for calling the theme silly. Ok, it is kinda funny. Actually, kmr is revamping it to something completely different, still red, which also involves changing structure of pages.
At that time, purple theme will not work properly anymore, so I won't point you to page to set the cookie. It is a beta theme by schminimalischt, though almost finished, I think. Haven't seen him around lately, so when kmr is done, I will make necessary changes if schmin isn't around then.
Probably a glitch with your cookies or something, all 3 work here, firebird 0.7. Plan changed, gonna finish restructuring and adapting schmin's theme at same time, you'll have some real options in a couple of weeks, I reckon.