To create or edit your app theme simply go to the Settings menu and select "App Settings".
Then you will need to choose "Themes."
Choose the group that you would like to Theme. If you have not created Groups, the Default selection will be your home page theme until you create more groups.
From here you will be able to choose a color, logo, and upload a graphic for your app home page. When you are finished creating your theme be sure to save your changes.