.fotorama__wrap--slide .fotorama__stage__frame {
opacity: 1 !important;
background-color: #FF0;
}
Find this classes in CSS, and add your bg-color rule... Btw, you can use developer tools in firefox, chrome, ie... (F12 will open it) and inspect element(s). That way, you can check/debug html/css/js easily... Note: this is solution for latest version of fotorama, not sure that class names are same in previous versions...