You can add Qt themes to Grabber, using CSS to customize all parts of the UI.
To add a new theme to Grabber, just create a folder in the
themes directory (located in
C:/Users/%USERNAME%/AppData/Local/Bionus/Grabber on Windows) with the name of your theme. Then create a
style.css file inside that new folder. You just have to write your CSS into that file!
The documentation for Qt's stylesheet can be found here: https://doc.qt.io/qt-5/stylesheet-reference.html
If you made a nice theme that you feel could be added to the default Grabber installation, feel free to propose it to the team by making a pull request!