If you go to the Edit menu and open the "Options" you should have an option for the "Topside Ruler Colour".
Is that set to White (255,255,255)?
If not, click on it, and then click on the 3 ... that appear to select a new colour.
Pick White from the center and click ok to confirm and close the options.
Switch to the 3D View and back to the 2D View to reload the Rulers with the new colour.
I hope this helps