mobile menu bar semi-transparent background

My main navigation links are over a background image on my site. I like how they look in the regular view. But when I look at the handheld view they are a little difficult to read. It would help if they had a transparent background color behind them to make them pop out, but ONLY in the handheld menu. Gray color would be fine, I just still want to see my regular background image behind the transparent background.

Is there a way to do this without disturbing the look of the non handheld view? Sorry I hope I explained this well enough. I know where to add extra CSS if that helps … just have no idea what code I need.

Thank you,
Karen