I believe those files do not refer to the system grid or the combat grid, but rather to grids elsewhere in the GUI, such as the galaxy map. As far as I know, there is no way to change the color of the grids on the system map and the combat map.
You could try an alternate Starmap.bmp image which provide greater contrast. Make sure to replace both the one in Pictures\System and in the appropriate subfolder of this folder, depending on which resolution you use. The small one in the system folder is used in combat and as the image in the small picture box on the right side of the screen, where planet images normally go for the selected planet. The ones in the 1024 and 800 folders are used as system backgrounds for the main game screen.