Backgrounds, fonts, and icons cannot be changed by custom themes with this release. Look for this capability in future releases. In addition to the above features, you can now apply customer colors to entities which show up in the navigation tiles (as well as other places). Microsoft is really working to improve MS CRM and make it more feature rich. Look for this update to be applied to the MS CRM 2015 online system in April 2015. Personalized Themes in Microsoft Dynamics is a great way to feel like an extension of the business. For the latest on themes, check out Microsoft Docs.