Currently this way has been implemented for wip/wx build.
Where to get? Go to Configs-> Setup and check for the popup which will be opened then;)
Since the upcoming 1.6dev1 version, you won't have to use that Setup window - it's used for the first-run (idk if I can do it), you will use the general Preferences window instead. It's located on Settings menu and can be triggered by pressing Ctrl+P.
INI way
First, look for basic components here. Also look for the INI file format basics.
If you use texteditor 1.5+/textworker 1.6a2+, application UI configurations are placed in ~/.config/textworker/themes/. For older versions, you can check this instead.
And...
You completed your changes? Restart the app and enjoy!