The blue background that you see in the demo is actually an Image. If you look in your theme files you’ll see the background image that is being used. You could either swap that image out with another image of a solid color, or you can add CSS to override it.
.front-page-3 {
background: none !important;
background-color: #1794ff !important;
}
The above code should override it if you don’t want to adjust any of the other code in the PHP or CSS files.
Let me know if that works.