change modal overlay color for client consistency

bigbluebutton-html5/imports/ui/components/modal/simple/styles.scss

@@ -41,5 +41,5 @@
 
 .overlay {
   @extend .overlay;
-  background-color: rgba(0, 0, 0, .5);
+  background-color: rgba(6, 23, 42, 0.75);
 }