Hi Peter,
seems you have the same problem as me
@dale made a comment on my post Dark/Light theme GUI changes - #2 by dale and suggested to derive all static texts (and i assume also your group boxes) from CRhinoUiStaticEx or at least CRhinoUiStatic
Unfortunately this leads to a lot of work changing all those static text fields (same on my project).
Dale also mentioned this problem is well known at least since V7, but he did not say anything if it’s reported or planned to fix.