The windows/beta of RC7 now has support for App theming by clicking on the "Themes" button in the FormBar.
Currently I have put together some basic themes for Default, Android, iOS and Modern.
You can find these in the \lianja\themes directory. Each theme has a .lianjatheme extension. You can customize your own themes by copying one of these files and giving it a meaningful name. When the App Builder starts it will add your own custom themes to the "Themes" menu. This is where you specify the css file also.
This is still a work in progress but most near all of the UI can be themed across a complete App now.
I will be adding some additional functionality into this over the next few days. My objective is to allow you to have a different theme based on the client type, so for example, when your App runs on an android tablet or an iPad the theme will be chosen based on the client.