Index: src/wp-admin/css/themes.css
===================================================================
--- src/wp-admin/css/themes.css	(revision 38516)
+++ src/wp-admin/css/themes.css	(working copy)
@@ -1182,6 +1182,8 @@
 
 body.full-overlay-active {
 	overflow: hidden;
+	/* Hide all the content, the Customizer overlay is then made visible to be the only available content. */
+	visibility: hidden;
 }
 
 .wp-full-overlay {
@@ -1613,8 +1615,10 @@
 	height: 100%;
 }
 
-.customize-active #customize-container {
-	display: block;
+/* Make the Customizer and Theme installer overlays the only available content. */
+#customize-container,
+.theme-install-overlay {
+	visibility: visible;
 }
 
 .customize-loading #customize-container iframe {
Index: src/wp-admin/js/customize-controls.js
===================================================================
--- src/wp-admin/js/customize-controls.js	(revision 38516)
+++ src/wp-admin/js/customize-controls.js	(working copy)
@@ -3620,7 +3620,7 @@
 			return;
 		}
 
-		var parent, topFocus,
+		var parent,
 			body = $( document.body ),
 			overlay = body.children( '.wp-full-overlay' ),
 			title = $( '#customize-info .panel-title.site-title' ),
@@ -4234,14 +4234,6 @@
 		});
 
 		api.trigger( 'ready' );
-
-		// Make sure left column gets focus
-		topFocus = closeBtn;
-		topFocus.focus();
-		setTimeout(function () {
-			topFocus.focus();
-		}, 200);
-
 	});
 
 })( wp, jQuery );
Index: src/wp-admin/js/theme.js
===================================================================
--- src/wp-admin/js/theme.js	(revision 38516)
+++ src/wp-admin/js/theme.js	(working copy)
@@ -866,9 +866,13 @@
 	html: themes.template( 'theme-preview' ),
 
 	render: function() {
-		var self = this, currentPreviewDevice,
-			data = this.model.toJSON();
+		var self = this,
+			currentPreviewDevice,
+			data = this.model.toJSON(),
+			$body = $( document.body );
 
+		$body.attr( 'aria-busy', 'true' );
+
 		this.$el.removeClass( 'iframe-ready' ).html( this.html( data ) );
 
 		currentPreviewDevice = this.$el.data( 'current-preview-device' );
@@ -879,8 +883,7 @@
 		themes.router.navigate( themes.router.baseUrl( themes.router.themePath + this.model.get( 'id' ) ), { replace: true } );
 
 		this.$el.fadeIn( 200, function() {
-			$( 'body' ).addClass( 'theme-installer-active full-overlay-active' );
-			$( '.close-full-overlay' ).focus();
+			$body.addClass( 'theme-installer-active full-overlay-active' );
 		});
 
 		this.$el.find( 'iframe' ).one( 'load', function() {
@@ -890,6 +893,7 @@
 
 	iframeLoaded: function() {
 		this.$el.addClass( 'iframe-ready' );
+		$( document.body ).attr( 'aria-busy', 'false' );
 	},
 
 	close: function() {
Index: src/wp-includes/js/customize-loader.js
===================================================================
--- src/wp-includes/js/customize-loader.js	(revision 38516)
+++ src/wp-includes/js/customize-loader.js	(working copy)
@@ -183,11 +183,11 @@
 		 * Callback after the Customizer has been opened.
 		 */
 		opened: function() {
-			Loader.body.addClass( 'customize-active full-overlay-active' );
+			Loader.body.addClass( 'customize-active full-overlay-active' ).attr( 'aria-busy', 'true' );
 		},
 
 		/**
-		 * Close the Customizer overlay and return focus to the link that opened it.
+		 * Close the Customizer overlay.
 		 */
 		close: function() {
 			if ( ! this.active ) {
@@ -209,11 +209,6 @@
 			if ( this.originalDocumentTitle ) {
 				document.title = this.originalDocumentTitle;
 			}
-
-			// Return focus to link that was originally clicked.
-			if ( this.link ) {
-				this.link.focus();
-			}
 		},
 
 		/**
@@ -227,6 +222,13 @@
 			Loader.saved     = null;
 			Loader.body.removeClass( 'customize-active full-overlay-active' ).removeClass( 'customize-loading' );
 			$( window ).off( 'beforeunload', Loader.beforeunload );
+			/*
+			 * Return focus to the link that opened the Customizer overlay after
+			 * the body element visibility is restored.
+			 */
+			if ( Loader.link ) {
+				Loader.link.focus();
+			}
 		},
 
 		/**
@@ -233,7 +235,7 @@
 		 * Callback for the `load` event on the Customizer iframe.
 		 */
 		loaded: function() {
-			Loader.body.removeClass('customize-loading');
+			Loader.body.removeClass( 'customize-loading' ).attr( 'aria-busy', 'false' );
 		},
 
 		/**