Seeing that the themes are XML files would it be possible to handle theme variants with a slider from dark to bright? Or have brightness & contrast sliders?
That way, you can tweak each theme a little by scaling the internal hex values of the theme without going in having to tweak each individual setting. The default theme could both have a darker variant and lighter one without having to create you own custom theme.
Another option might be to define an overall color palette with a few primary colors and have the theming engine handle all the small differences for sub panels, headers, etc.
The reason I bring this up is because it’s extremely tedious right now to try and find the right value to tweak.
For example, if you want to change the header color of all your panels you have to go through each one and change them individually, which seems like something that could be simplified.