By default, the “No listings were found matching your selection.” notice appears on a blue background when you are searching for a given key phrase and nothing relevant is found.
You can change its background to anything of your choosing using the CSS code below:
.bsui .alert-info {
color: #fff;
background-color: #17a2b8;
border-color: #17a2b8;
}CSS
You will need to place the above code at WP > Appearance > Customize > Additional CSS. You can replace the color values with any other color that you prefer.