Mobile Entree Themes provide a means for you to develop a custom look and feel for your mobile users that is packaged and deployed using SharePoint's solution/feature provisioning system.  A theme contains resources that Mobile Entree uses for rendering.  These resources are merged with the out-of-the-box resources to allow the developer to only change the things that he requires.

Some of the files that you can register in a theme include UserAgentMapping.xml, ListStyles.xml as well as your own CSS files.

This Video walks you through creating your own custom theme: http://itsbestwith.me/listing.aspx?id=17