Does your site have everything?

[Techdomain]

same deal with IPB... there is just no system that would work properly as suggested unless all themes in it were text based clones of eachother... I guess then for a "different" look you could change css

[akulion]

The one thing I remember about IPB is that they had a 1000 skins which all looked nearly the same in design but different colors

But I must say that adding stuff to IPB was really simple, just edit the wrapper and viola its on all themes.