For this example, we will be using "DefaultClean" theme.
1) Go to your theme folder in this location: Nop.Web/Views/Themes/DefaultClean
2) Now, go to the style.css which is located in this location:
Themes/DefaultClean/Content/style.css
3) Open the style.css file and look for the class "header-menu"
4) Once you find the class, you will see this code:
/* top menu */
.header-menu{background:none repeat scroll 0 0 #404040;border-radius:4px;-o-border-radius:4px;-icab-border-radius:4px;-khtml-border-radius:4px;-moz-border-radius:4px;-webkit-border-radius:4px;-moz-border-radius:4px;height:auto;list-style:none outside none;margin:10px auto 0;padding:0 10px;}
5) You can see in the code, the background color is defined as: #404040
6) For this example, we will change the color to blue (#3104B4) like this:
.header-menu{background:none repeat scroll 0 0 #3104B4;border-radius:4px;-o-border-radius:4px;-icab-border-radius:4px;-khtml-border-radius:4px;-moz-border-radius:4px;-webkit-border-radius:4px;-moz-border-radius:4px;height:auto;list-style:none outside none;margin:10px auto 0;padding:0 10px;}
7) That's it - Run your website and see the updated background color of your top menu bar:
Note: In case, you do not see any change, make sure to clear the history and cache of your browser.