scale the background for large monitors

htdocs/css/openwebrx.css

@@ -251,6 +251,7 @@ input[type=range]:focus::-ms-fill-upper
 	background-image: url('../gfx/openwebrx-background-cool-blue.png');
 	background-repeat: no-repeat;
 	background-color: #1e5f7f;
+	background-size: cover;
 	cursor: crosshair;
 }