Chiyoko Yoshida of chiyofm created a handy little file to help you visualize the changes to layout objects when you apply different themes. Each layout contains all the objects you could possibly style on your FileMaker layouts, such as buttons, tab and slide panels, portals, text, and more. You could use this as a base for a theme development file (see my previous post about working with themes and styles).
All you need to do is duplicate a layout and either apply an existing theme, or use it as a base to create a custom theme. Either way, the work of adding all the possible objects has been done for you! Thanks to Chiyoko for making this available!