Hi there, if you are talking about the site title/nav bar, then the background color for that is set in #masthead-wrap
#masthead-wrap {
background-color: #000;
}
If you have not yet done so, I would suggest creating a child theme so that any customizations you make will not be overwritten by a future theme update. Alternately you can use the Edit CSS function in the Jetpack plugin, or use one of the other Custom CSS plugins that will keep CSS changes from being overwritten.
Child Themes
Child Theme creation plugins
Jetpack plugin
Custom CSS plugins