Only an issue on 10022

This has to do with the glass windowframe we're using, and the use (or lack thereof) of exclude-glass, I expect. See bug 1169911 comment 1.

We probably need a new grid image when we change the background color to remain white, though - white on white won't be very visible. :-)

Stephen, can you clarify how you want it to look?

Assuming we need to do something here, P3 (not super-high-prio because people don't spend oodles of time in customize mode and the current state doesn't interfere heavily with functionality).

Seems fixed on my VM. Haven't tried on my windows 10 laptop yet.

This looks fixed on Nightly with the latest Windows 10 build (10162). Background is the default white of the window. Using Gij's patch from bug 1173725 appears to do the right thing by making everything a darker grey.