Sometimes you need to change the color of the menu in the mobile layout (the 3 lines called hamburger). And sometimes you want to add a sharp, for example a circle, in the background to avoid problems with colors.
The final effect you want is something like this:
If you try to change the color you find a problem, because the three lines in Divi use !important to set the color, so if you try to override the color you fail.
A method I found to change the color is to work on the specific element:
span.mobile_menu_bar:before {
color: #000000 !important;
}
Then if you want to add a circle in the backgorund you have to add:
.et_toggle_fullscreen_menu {
background: #fff;
-moz-border-radius: 50px;
-webkit-border-radius: 50px;
border-radius: 50px;
}