I don’t think it will be quite as simple as dropping in a new css. There will be a number of other tweaks necessary to get a consistant look for the forum & control panel.
But yes the power of css is enormous. Its good to have a separate css to handle print and mobile friendly layouts as well (ie low graphics single column).
Have a look at the expression engine Showcase, for more inspiration of what people have done running on this platform.
Its been a while since I’ve done any web development but I’ll try and have a go at one when I’ve finished my project.
