LOL ... its so funny you asking this...
Because today I ran across the same problem. However, I did not change the background of the map window i changed the color of the lines that I could not see.
Since I dont know how to change the background of the map window and have never had much luck with mod requests here, I thought I would try to fix it myself.
Here is how I fixed it:
I had a problem seeing the lines for the plane of Nightmare map.
Here is a small sample of what you will see in a map file (ponightmare_1.txt for this example).
L -305.0000, 2414.0000, 169.0000, -366.0000, 2414.0000, 169.0000, 150, 150, 150
L -305.0000, 2385.0000, 169.0000, -305.0000, 2414.0000, 169.0000, 150, 150, 150
L -366.0000, 2385.0000, 169.0000, -305.0000, 2385.0000, 169.0000, 150, 150, 150
The last 3 numbers represent the color in RBG values. So the value there (150, 150, 150) is a light gray. These were the lines I could not see.
So to change the color go into Notepad and open the file. Then click on Edit in the file menu and click on Replace.
When the replace window comes up type the color code that you want to find (this case the light gray 150, 150, 150). In the replace area type the color you want the lines to be (I chose a dark gray, almost black color 73, 73, 73). Click Replace All and save the file.
So the corrected lines would look like this:
L -305.0000, 2414.0000, 169.0000, -366.0000, 2414.0000, 169.0000,
73, 73, 73
L -305.0000, 2385.0000, 169.0000, -305.0000, 2414.0000, 169.0000,
73, 73, 73
L -366.0000, 2385.0000, 169.0000, -305.0000, 2385.0000, 169.0000,
73, 73, 73
Tada ... easy to see lines.
I am sure changin the background will help .. bu eventually some color is not going to be seen very well on whatever background you choose. This is at least something good to know.
Hope it helps
-Reed