add the code below at the end of style.css
.middle-menu {
background-color: blue;
}
you will find style.css at EDITOR at the end of Appearance tab https://i.imgur.com/bWIKSUK.png
for other colors, go to https://html-color-codes.info and pick your color and replace blue with the html color code.
background-color: #000; // for black
