Hi Ismo
Okay, please add the CSS code below in the Additional CSS Box. Please go to the Appearance => Customize => Additional CSS then paste the CSS here and click to Publish button.
.navbar {
background-color: #dd3333 !important;
}
body.page-template-frontpage .navbar {
background-color: transparent !important;
}
body.page-template-frontpage .navbar.header-fixed-top {
background-color: #dd3333 !important;
}
You can change the color code as per your requirement.
Thanks