You can change theme in your profile settings.
Okay, now I see it, you have to log on to the forum, then click on "Profile", then hover the cursor over "Modify Profile", then click on "Look and Layout", then locate "Current Theme:" and then click on "(change)".
So, that means I first have to change it and then be logged on to the forum all the time to see the new theme.
Why don't you just make it the new default for the forum?