Martin -
You will probably want to take a look at Library Projects to keep your codebase clean. A common
pattern is to create a Library Project that has 90-100% of your
Java code and a set of default resources for your themes and styles. You can then create multiple deployable application projects that utilize the library that simply override the resources (XML files and drawables) that you need to create a new theme or provide a different name, etc. for the application. Each deployable application will have its own package name, so that they can all be uploaded to Google Play, but the projects themselves will contain very little code so you shouldn't have too many issues with duplication.
More about Library Projects can be found on the Android Developer Site:
http://developer.android.com/tools/projects/index.html#LibraryProjects