[Resolved] Change menu background color in mobile mode

This is the custom CSS you need to change these colors, and add a border.
Add it to ‘Appearance’ > ‘Customize’ > ‘Additional CSS’, changing the color values to the color you want.

@media (max-width: 1024px) {
    /* drop down menu */
    #site-navigation {
        background: #f9f9f9;
        border: 1px solid #000000;
    }

    /* sub menu within the main drop down */
    .main-navigation ul ul {
        background: #f6f6f6;
    }
}