My Ruler Background is custom colored in light grey (V: 60), which in turn gives the lines in black color. Once I open/close Preferences window the lines in Ruler turn white.
To fix the issue I have to open the Prefs window again, go to Color Schemes and change the Ruler Background color two steps lighter (V: 62) then click ok, then click apply, then go back to color picker again, but now it shows that V is 61. So, now I have to change brightness one step down (V: 60) then one step up (V:61), the click ok. Now if I check Ruler Backrgound color, it is once again V: 60 and the lines are black.
This is the only way to fix this and have to do this every time I open/close Prefs window.