Alternating colours at li

[PcPostar]

Hi, i have a navigation menu on the left, and I would like to alternate background of list items.
So that 1, 3, 5 entry of menu will have dark background and 2, 4, 6 entry of menu will have a light background.

I could manage this with css, but all li are in the same class. Is it however possible in simple way to alternate those classes of <li> at menu....

thanks