Would you mind trying this code to resolve it under your Appearance > Theme Options > Custom CSS:
background: #eeeeee !important;
And in another way you can go to the Appearance > Theme Options > Styling > Or custom menu color as per shown here: http://prntscr.com/7fpdu5
You can only use either way between both above mentioned. Or the third way is to go under Styling > Disable Menu Background
Hope this helps!