These themes can be easily created and edited with SimpleEdit itself or you can use any other Plist Editor. The themes are stored in a sub folder of its 'Application Support Folder'.
This can be found in the Mac App Store version here: ~/Library/Containers/com.myownapp.SimpleEdit/Data/Library/Application\Support/SimpleEdit/Themes
And in the Non-MAS version here:~/Library/Application\Support/SimpleEdit/Themes
These themes are 'loaded' every time you start SimpleEdit or when you create a new document. A theme is nothing else than a dictionary with objects and keys - much like JSON. It is based on RGBA colors most of us already know from CSS: color: rgba(255,255,255,1.0);
You can download the default theme here
and you can create as many themes as you like to and apply them in the Preferences.