In this video I walk through all the specifics related to customizing one of the existing themes. Even still, if you can't take the time to learn every single detail, you can still work with a theme which currently exists and simply tweak that theme to fit your specific needs. The trick is knowing what's required, what can and can't be done and how the CSS translates to the FileMaker UI widgets. In the time since that initial release it may not be as commonly known, but the location still exists and you can certainly create (or modify) your own theme. When FileMaker, now Claris, first introduced themes they likely had the intention of allowing users and/or third parties to create their own.

They can be seen when extracting them from the clipboard, and if you know the right location to retrieve and store them, can be defined by any enthusiastic developer.

Each element on your FileMaker layouts is defined by these CSS settings. FileMaker themes are simply a collection of the popular CSS standards plus a few extra attributes which are FileMaker specific.

Filemaker pro 15 themes