Fixes shared video thumbnail size and shared video on large background.

css/videolayout_default.css

 
 .sharedVideoAvatar {
     height: 100%;
+    width: 100%;
+    object-fit: cover;
 }
 
 .noMic {

modules/UI/shared_video/SharedVideo.js

     }
 
     show () {
+        let self = this;
         return new Promise(resolve => {
             this.$iframe.fadeIn(300, () => {
+                self.bodyBackground = document.body.style.background;
+                document.body.style.background = 'black';
                 this.$iframe.css({opacity: 1});
                 resolve();
             });
     }
 
     hide () {
+        let self = this;
         return new Promise(resolve => {
             this.$iframe.fadeOut(300, () => {
+                document.body.style.background = self.bodyBackground;
                 this.$iframe.css({opacity: 0});
                 resolve();
             });