CSS styling mobile toggle menu

I have been using the Additional CSS box in customise to adjust my nav bar. I am happy with the way it looks on desktop but I cannot work out how to change the hamburger menu colour and the background colour when toggled.

This is the CSS I have added so far:

#masthead,
#site-branding {
background-color:#060026;
padding:10px;}
#primary-menu li.menu-item a {
color:#ffffff;
}

Any help appreciated. Thanks.

The page I need help with: [log in to see the link]