~/usr/share/themes/, find the folder for the theme that you want to change (Greybird-dark in my case) and edit the themerc.txt file in that folder in a text editor opened as root.
The parameters that you want to set/change are detailed here. Replacing defaults where needed, I added
Once you are done, exit the text editor. Then, change your theme to something else and then change back to the one that you've just changed. That should activate your changes.
Note that these settings will sometimes be overwritten by significant upgrades. For example, I lost them to the Xubuntu 20.04 -> Xubuntu 22.04 jump.