Hints:
- Window > Themes > Save to file… (except for the Default Theme , Use White instead)
- Unzip the .scrtheme file with 7-zip (don’t bother to change the extention to .zip)
- You’ll find four files: *.pal, *.prefs, *.xml and… *.qss.
- QSS is a variant of CSS, add your code to it with a text editor (I use MS Code)
- Zip the four files with 7-zip and rename to .scrtheme
- Window > Themes > Load from file…