Thank you Michael for the constructive input. I also prefer the no-skin GUI of Xara and think that the introduction of the proprietary black GUI that can not be customized even with respect to menu font size is a historic, epic management mistake (my guess is that some marketing guys at Magix who have never used any of their programs themselves are to blame for this).

Traditionally, only infantile overclocking apps for the graphic cards of 15-years olds are designed this way.

Anyway, and apart from that, unfortunately the toolbars of Xara's non-skinned GUI are even more broken (e.g. even tinier) than with the dark skin on HiDPI screens, see below screenshots:

Skinned GUI:
http://i.imgur.com/UaQOtCC.png

Native GUI:
http://i.imgur.com/5bPj4v9.png

Note especially how the numbers in those microscopic input boxes are totally unreadable.