In an (almost) dramatic diversion from the past, the latest build supports a fairly complete theme colour control, along with a rather extreme demo of what is possible. It is almost complete, but as it has no effect on the actual sound, I'll only be giving it a minor point release.
At the same time I've tried to make button styles and colours more consistent, and clearly separate the idea of action buttons that do something and link buttons that take you to another place.
Finally, all buttons/switches/counters/menus that are MIDI-learnable now have bold italic blue text. They are more obvious that way. As before, all sliders and all knobs (except master fine detune) are learnable.
However, this all needs more eyeballs on it, and also other people to experiment with creating variations on the existing ones or completely new ones from scratch. The structure is quite heavily error checked and nothing should actually break. When a fault is found the line number is reported and no other entries in that file will be attempted.
Oh, and I recommend setting your gray scale first. It has quite a dramatic effect on everything else!
These are plain text files held in /home/{user}/.local/share/yoshimi/themes
Theme management is done via a new 'Theme' tab in Settings (via the Yoshimi dropdown). Changing theme is pretty much instant
'classic' is an example file and the actual classic theme can't be changed.