in the same file, look for:
/* If it fits I sits... */
.navigate_section ul, .popup_content, .up_contain {
background: #fff;
background-image: linear-gradient(to bottom, #fff 0%, #f1f3f5 95%);
That section is a linear-gradient which means it's two colors mixed in top to bottom. With the background-image line, the first color is the top part and the second color is the bottom. You can make the whole thing one color, or change the percentage, etc.
Play around with it until you get the look you want. If you want just one color, you can also get rid of that line altogether and just change the background color to whatever you want