I have seen several posts in the forums recently about styling or theming CRM 2011. I just want to mention a few things about this and the role of the CRM 2011 Style Guide included in the SDK in the sdk\resources\styleguide folder. I was surprised at how little documentation I could find about it's use outside of the e-learning training for the customization Exam for CRM 4.0
What it is:
The styleguide is a collection of CSS style sheets and diagrams showing what styles, fonts, spacings, etc... are used within the CRM application. This allows a developer or designer to recreate the look and feel of the CRM application in custom applications that may be integrated with CRM. This gives the user a more seamless experience that looks congruent with the experience you get from working within the CRM application.
What it isn't:
A way to style or theme the CRM application itself.
Most custom styling that you can accomplish through any code means (other than using explicit settings in the GUI) is unsupported and could affect the way CRM functions. These types of customizations could be blown away by a rollup update or future upgrade; or worse, if a rollup or upgrade changed the way the CRM page structure functioned, it might break your CRM form. I have also seen people who have wondering about supposed themes for CRM, the two people I have heard this from were really looking at Outlook themes that can be used in Outlook (which may or may not have the CRM for Outlook client installed). There is no supported way to "theme" the application.
I hope this helps!