How to I change the color on the background of a dropdown?

Does anyone know how how to I change the color on the background of a dropdown?  Thank you!

You can add this to Design > Custom CSS

/* Dropdown Color */
.header-nav-folder-content {
    background: #f1f !important;
}

Thank you @tuanphan! You are incredible.

So I got that to work but I have two questions now.

1) Is there a way that I can change the opacity so the background doesn't look too solid?

2) Some of the pages have a lighter background so is it possible to not use the dropdown background for those pages? Just keeping those pages clean