Change background color on DropDown mobile version

Hello
I installed the JUX CSS3 MEGA MENU on my site.

I applied the blackwhite theme. I change the firs level color to a black transparent color in blackwhite.css.

The problem is that, when i acces my site by mobile, when i press that three lines, open my dropdowmn menu, the color is that transparent color editted above.

How can i change the color for dropdown menu only on mobile version. How can i leave the first level the same as desktop view and change only the backgroun color for dropdown menu?

Here is what i want to do: ->>> catalinstaicudesign.ro/images/m1.png

Can you please healp me?

Best regards,
Catalin