See those three lines in the top right in the image below? That's called a hamburger menu. No, you don't use it to order a juicy burger, but it's pretty vital to your mobile website. That menu holds your navigation that would normally stretch across the top or side of your website.
Sometimes when editing your Portfolio, you may inevitably edit your background color to the same color as your mobile "hamburger" menu that is only visible when viewing your site on a mobile device. To change the color of the menu to make it stand out, click the pencil icon button in your Navigation section and click "Customize". In the Navigation Container menu, look for "Mobile Icon" and click the color picker.