You need a new style rule for div.nav-top.
In order to do this, the first thing you will need, to be sure that updates to the theme you’re using (mystique?) don’t overwrite your changes, is a Child Theme. Just create a minimal child theme that behaves exactly like its parent. You will just need a style.css file with the required comments and an @import statement referencing the parent style sheet.
Once you have done that, you can add a style rule like the following to the end of the child theme’s style sheet:
div.nav-top {
background-color: /* your colour value here */;
}
HTH
PAE
.