Yes, I did.
I was talking about the top right three buttons in standard window frames, nothing else. All I did was resize the two caption bar images in paint.net for Caption and Caption Glass, then resizing the "OS caption bar" value in "Extra settings" accordingly.
Still, when I load the original Diamond theme, every button in the top right of the caption frame reacts on its own at mouseover, whereas in my versions of it, all three light up if I touch only one of them. And they stay lit up until I mouseover the caption bar somewhere else.
EDIT: I redid everything in version 11 of SkinStudio, and now everything works as it should. So, I'm guessing, it was simply a bug in the software.
Thanks for helping,
Mubuk