i've done a hack lately, so for each usergroup, you have more than 3 banners set.. header/navbar/footer ... you can integrate the code in all your styles, so you don't have to modify the templates each time...

the client is very happy with the solution because you simply have to enter the html tag for the banner you want to display for each usergroup, instead of making conditionals for each template...

just PM me if you are interested... (small fee btw)

hum.. but i read the request again... selecting what usergroup can use a skin, may be another kind of hack... interesting feature btw... a skin by default for each usergroup, and "this skin can be used by XX usergroups ... two good features to add... may have to take a look myself if a client come to ask.