You can modify background color under main menu using basic CSS. Here is default code, please override vaules to customize appearance.

#sp-header {
 background-color: rgba(255, 255, 255, 0.96);
 box-shadow: 0 3px 3px rgba(0, 0, 0, 0.05);
 height: 60px;
 width: 100% !important;
 }

We suggest to use custom.css file for it. This file have to be created in /css folder of template.