Go into WindowBlinds Config
Click on Colours tab on bottom
Click on "Change Skin System Colors" button on lower right
In "Select a substyle to edit" dropdown, click on the substyle you want to change
In "Select a colour to edit" section, scroll down to "System Colour:Window" (almost to bottom) and click on it
Click on "Change colour" button on lower left
Select a lighter color from the color picker window
Screenshot for a similar example in Reply 1 here: https://forums.wincustomize.com/400010 (shows for "ButtonFace" though, which is NOT what you are wanting to change).