Dear Frederic,
I would like to confirm what you are referring to as "menu top background", are you referring to the attached screenshot (inside red box)?
If yes then you can change the default gray background by following the steps:
1.) Go to Appearance — Theme Options.
2.) Click "Custom Code".
3.) Check "Enable Custom CSS" and make sure "Load the theme CSS" option is selected.
4.) Add the following CSS code in the text area:
.main-navigation{background:#000 !important;}
Change the background color to anything you like. I use black (#000) as my example.
5.) Click "Save Changes". See attached "after_editing.jpg" as to how it looks like after saving the changes on the theme options panel.
In this method, you can add some custom CSS code if they are not yet included on the theme options background and font color options.
Please let me know how it goes.
Cheers,
Emerson