I like that the themes are not simply a new color scheme and logo. I love your use of color and bold graphics. I noticed some things you may want to look at...
In Mozilla Firebird, the summer theme has a slight hitch. The beach "layer" in the background goes up above your three content columns by about 20 pixels. It's not glaring, but, is noticable.
In IE 5.5, the summer theme has other problems. The header image is shrunk down and shoved to the upper right. The orange "beach row" drops below that overlays your content.
You may find this site handy for your future testing. A quick way to show your clients how their site will look across all the different browsers out there:http://www.browsercam.com/