2.0 Themes at Dev Time

I have several themes that I got from various places. They all consist of a
..css a .skin and various images. But only one of the 5 themes takes effect
at development time. The other four only take effect at run time. I've
compared them and I can't see what it is about the one the works at dev time
that makes it different from the others.

I'ts a lot easier to design the pages when I can see the theme taking effect
at design time.

Does anyone know what it is that causes this?

Thanks,
T