What he means is Office skins itself so will not pickup on the WB skin in many places.
However it will pickup on the system wide colour scheme which is part of a skin. This is not something that can be excluded from an app as the Windows colour scheme is global.
In this case it sounds like the skin you are using has a grey window colour Vs white which is why you are having a problem.
Using another skin that's using white for a window colour would avoid your issue.
In short this is a skin issue not a WindowBlinds one.