It looks like the theme developer used background images for the navigation shape and color rather than css3. One of the easier ways to change the color in this case would be to edit the image file they used. It is located at this file path
Download and open it. You should see the images that make up the nav. If you can change the color of the green, it should work if you reupload the editted graphic. The trick is to place the replacement images for the green into the exact same places. If it's off the nav graphics will be off. If off by a little though, you could edit the css background property that shifts it up/down, left/right a smidge.