If you like getting your hands dirty go over to either: https://sourceforge.net/projects/yoshimi
or: https://github.com/Yoshimi/yoshimi an download and build the current master.
This still isn't quite ready for release, but has a new way of handling themes that could do with a good workout.
First of all, changing themes is now much faster and without any flicker.
Secondly setting 'Monochrome' takes effect immediately, and doesn't require a re-load.
The most significant new feature is that any time the Theme window is visible, the currently loaded theme is checked against the stored one every 500mS, and if the stored one has changed it is reloaded.
This means it is possible to edit themes and immediately see the actual result. It isn't (yet) possible to directly create a new theme in place within Yoshimi - the simplest way to do it is to export one of the existing themes, then import it again - changing the name.
Next point a text editor of your choice to {user}.local/share/yoshimi/themes/{your new theme}
Then get twiddling the numbers ![]()