Source: https://www.elegantthemes.com/forum/viewtopic.php?f=187&t=514868

The mobile menu uses the global accent color for the hamburger and the separator line between the main menu and submenu’s. Use this CSS to change this to a different color:

/** -- START change mobile menu accent color  -- **/
/* -- Hamburger icon -- */
.mobile_menu_bar::before, 
.mobile_menu_bar::after{
color: #ffffff;
}
/* -- Line color above mobile menu drop down -- */
.et-search-form, .nav li ul, .et_mobile_menu, .footer-widget li:before, .et_pb_pricing li:before, blockquote{
border-color: #ffffff;
}
/** -- END change mobile menu accent color  -- **/
Code language: CSS (css)