As you might of seen, the themes option is removed in Exchange 2010.. the old layout has been changed to some ugly yellow stylish look (probably to make it look like Office 2010.. ). Personally I do not like the yellow and have always used the Zune theme in Outlook 2007 (not that I like the logo, more the coloring).
When you install the CAS role, you will notice that the OWA folder (that holds all the owa pictures) actually still has multiple folders.. 1,2,3 and base. And that base is the normal OWA design. 1 apparently is some dark neon glow sort of theme, 2 is the Xbox 360 theme and 3 is a fully renewed Zune theme!
Now since the option to choose a theme has been removed, an administrator can only set a theme for everybody (but way better than the dodgy yellow). Create a copy of the base folder and copy the contents of 1, 2 or 3 in the base folder.. press shift F5 on your webbrowser and voila a new theme!