Hello,
please open your style.css file
(you can find it here \wp-content\themes\revelance\style.css)
and on the line 682 you have this code:
#abdev_main_header.menu_over_slider {
background: none;
-webkit-box-shadow: none;
box-shadow: none;
}
Change background: none;
in color that you like :)
Let us know if you need anything else.
Kind regards