For the menu styling, this is located in the theme’s /css/menu.css location of your theme files. The section that styles the mobile menu begins around line 115 and starts with this:
@media (max-width: 979px) {
All your colours will be found there but you can override these if using a child theme by adding this to your child theme style.css file:
@media (max-width: 979px) {
your overrides here
}
Otherwise, if you plan to use the original menu.css file to make changes, the above info will help you locate it.