Here is how it goes:
Go to Advanced>>>Game Interface, and scroll down until you see the Image section.
Now find these:
Image - Cinematic Border
Image - Console Inventory
Image - Console Inventory None
Image - Console Texture 1
Image - Console Texture 2
Image - Console Texture 3
Image - Console Texture 4
Image - Game Menu Border
Image - Quest Dialog Border
You can now just double click and change their paths to something you like it to be.
Try just removing the paths, and see if they turned black. If not, you can look for some black square or something black in WE's MPQ, and if you can't find it, just draw a black square, convert it to .blp (You can do that using
Magos), import it into a map via Import Manager and change its path to the ones in Game Interface.
Here is some extra help on changing UI>>>
Click Me!
See if that does it...