Ok I just tested different style sheets in different boards with 6.1br1 and it seems to be working as I thought it did.
I assigned a forum a specific stylesheet (the blues) when I created it. (I can't edit a forum with 6.1br1 because of a bug)
All of the rest of the site is using the "infopop" stylesheet.
Provided my personal display preferences are set to "Always use default" then I see (the blues) when using the new forum but the rest of the site is still "infopop".
If you remove the stylesheet choices from the users then the only thing left should be "Always use default" in their display prefs. Now they will only see what you want them to see.

Or, am I still missing the boat on this... lol

Thanks about IIP too
