fix(background-alpha) Fix setting background opacity

modules/UI/UI.js

         $('body').addClass('mobile-browser');
     } else {
         $('body').addClass('desktop-browser');
+    }
 
-        if (config.backgroundAlpha !== undefined) {
-            const backgroundColor = $('body').css('background-color');
-            const alphaColor = setColorAlpha(backgroundColor, config.backgroundAlpha);
+    if (config.backgroundAlpha !== undefined) {
+        const backgroundColor = $('body').css('background-color');
+        const alphaColor = setColorAlpha(backgroundColor, config.backgroundAlpha);
 
-            $('body').css('background-color', alphaColor);
-        }
+        $('body').css('background-color', alphaColor);
     }
 
     if (config.iAmRecorder) {

react/features/conference/components/web/Conference.js

                 id = 'layout_wrapper'
                 onMouseEnter = { this._onMouseEnter }
                 onMouseLeave = { this._onMouseLeave }
-                onMouseMove = { this._onMouseMove } >
+                onMouseMove = { this._onMouseMove }
+                ref = { this._setBackground }>
                 <Chat />
                 <div
                     className = { clsx(_layoutClassName, _showStageFilmstrip && 'stage-filmstrip') }
                     id = 'videoconference_page'
-                    onMouseMove = { isMobileBrowser() ? undefined : this._onShowToolbar }
-                    ref = { this._setBackground }>
+                    onMouseMove = { isMobileBrowser() ? undefined : this._onShowToolbar }>
                     <ConferenceInfo />
                     <Notice />
                     <div