In the following screenshot one of ten checkboxes is selected, but it's impossible for me to tell which one. Only probing with the space bar helps me to find it out.
It seems that Mp3tag uses the same grey color as in light mode to draw a rectangle around the selected checkbox. Would it be possible to use the windows systems accent color instead?
As you can see I use "gold" as accent color, and that color gives a good contrast in dark mode.