@ I Am Pariah:
THANK YOU SOO MUCH !!!
I also found a way (because of your description) how to fix this problem without SkinStudio Pro. Since this is just ASCII Code editing I figured it should be plainly visible in the themes-folder. So for everyone who doesn't want to use (or have) SkinStudio Pro:
1. Go to the directory where WB saves your themes, e.g. C:\Programs\Stardock\Object Desktop\WindowBlinds\...
and then "\Noire" if the Theme is called like that.
2. open the "Themename".uis - file with the windows Editor or Wordpad.
3. search ("ctrl-F") the string "frame" until you are in the [color] section mentioned by I Am Pariahs description.
(found here : https://forums.wincustomize.com/?forumid=167&aid=93764 , or link above)
4. change WindowFrame colors to 41,41,41
5. it also helps if you change GrayText to 180, 180, 180 (a slightly darker grey than in the original theme. Couldn't read the damn pulldown menus properly before due to weak contrasts)
6. Save the file under the original name and apply the theme in windowblinds configuration.
Again Thanks To Pariah.
read U, sir steve.