All our kids rooms are neutral walls, carpets, drapes etc we just did the theme's using Wall Stickers and a rug for each room in that 'theme' which will need replacing as they get older anyway and now DD is in a bed we got her a couple of doona covers with butterflies on them.

The boys bed decor is more colour based as one boy is predominately blue and the other green - I did this so as they get older it is hopefully going to mean that they have a 'colour' which will designate who 'owns' what instead of having everything shared as we all like our own things