Apparently MDI windows don't change background color, which means that flat toolbars look bad with the white background instead of GetSysColor( 15 ). I did use the SetWindowColor( GetSysColor( 15 ) ); call, but it seems to do nothing.
Where did you use the call? On Create is too early to use SetWindowColor. MDI frame windows use whatever system setting you have for the color.
I figured it out, I thought that I could do everything in OnCreate, but after I moved it to main it worked.
You can do most things in OnCreate. But because that is called while Windows is still processing the CreateWindowEx call there are a few things you can't do. SetWindowColor is one of them. Although I can probably code around that.