Hi,
You can change the navigation bar background color via customizer(Theme Option). However if you need to change only for mobile devices you have to add the custom css just go to Appearance > Customize/Theme Options > Design Options Tab > Custom CSS paste the following CSS code and Click on save button. Do not forget to change the HEX color code #990099 below to your own color.
@media only screen and (max-width: 767px) {
#access {
background-color: #990099;
}
}
Thank you for using our theme.