Hello,

First, when finding the css to customize, do not ever see the backend. Check only front-end/live site. The css you pasted(if is exactly like that) has errors.
Try adding this one:

/*mobile*/
@media (max-width: 480px){
#logo img { height: 60px;}
.cl-mobile-menu-button span { background-color: #fff; }
.header_container * {
    background-color: #20263a;
}
}
/*tablet*/
@media (min-width: 1024px){
#logo img { height: 60px;}
.cl-mobile-menu-button span { background-color: #fff; }
.header_container * {
    background-color: #20263a;
}
}

Mobile result screenshot: https://ibb.co/0hbZKZB
Change the values for tablet, to your wish (i used the same as for mobile for this example).

Best regards!