Index: src/wp-admin/css/themes.css
===================================================================
--- src/wp-admin/css/themes.css	(revision 33527)
+++ src/wp-admin/css/themes.css	(working copy)
@@ -1280,6 +1280,7 @@
 
 body.full-overlay-active {
 	overflow: hidden;
+	visibility: hidden; /* hide content and make it not focusable */
 }
 
 .wp-full-overlay {
@@ -1585,8 +1586,10 @@
 	height: 100%;
 }
 
-.customize-active #customize-container {
-	display: block;
+/* Make the Customizer and Theme installer overlays the only focusable content */
+#customize-container,
+.theme-install-overlay {
+	visibility: visible;
 }
 
 .customize-loading #customize-container iframe {
@@ -1685,7 +1688,7 @@
 	border-bottom: 1px solid #ddd;
 }
 
-.theme-install-overlay .wp-full-overlay-main {
+.theme-preview-loading .wp-full-overlay-main {
 	background: #fff url(../images/spinner.gif) no-repeat center center;
 	-webkit-background-size: 20px 20px;
 	background-size: 20px 20px;
Index: src/wp-admin/js/customize-controls.js
===================================================================
--- src/wp-admin/js/customize-controls.js	(revision 33527)
+++ src/wp-admin/js/customize-controls.js	(working copy)
@@ -3041,7 +3041,7 @@
 			return;
 		}
 
-		var parent, topFocus,
+		var parent,
 			body = $( document.body ),
 			overlay = body.children( '.wp-full-overlay' ),
 			title = $( '#customize-info .panel-title.site-title' ),
@@ -3555,13 +3555,6 @@
 
 		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 33527)
+++ src/wp-admin/js/theme.js	(working copy)
@@ -720,6 +720,7 @@
 	nextTheme: function() {
 		var self = this;
 		self.trigger( 'theme:next', self.model.cid );
+		$( document.body ).addClass( 'theme-preview-loading' )
 		return false;
 	},
 
@@ -726,6 +727,7 @@
 	previousTheme: function() {
 		var self = this;
 		self.trigger( 'theme:previous', self.model.cid );
+		$( document.body ).addClass( 'theme-preview-loading' )
 		return false;
 	},
 
@@ -764,18 +766,30 @@
 	html: themes.template( 'theme-preview' ),
 
 	render: function() {
-		var data = this.model.toJSON();
+		var data = this.model.toJSON(),
+			$body = $( document.body );
 
+		$body.addClass( 'theme-preview-loading' ).attr( 'aria-busy', 'true' );
+
 		this.$el.html( this.html( data ) );
 
+		this.iframe = this.$el.find( '.wp-full-overlay-main iframe' );
+		this.iframe.one( 'load', this.loaded );
+
 		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' );
 		});
 	},
 
+	/**
+	 * Callback for the `load` event on the Theme installer iframe.
+	 */
+	loaded: function() {
+		$( document.body ).removeClass( 'theme-preview-loading' ).attr( 'aria-busy', 'false' );
+	},
+
 	close: function() {
 		this.$el.fadeOut( 200, function() {
 			$( 'body' ).removeClass( 'theme-installer-active full-overlay-active' );
Index: src/wp-includes/js/customize-loader.js
===================================================================
--- src/wp-includes/js/customize-loader.js	(revision 33527)
+++ src/wp-includes/js/customize-loader.js	(working copy)
@@ -181,7 +181,7 @@
 		 * 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' );
 		},
 
 		/**
@@ -207,11 +207,6 @@
 			if ( this.originalDocumentTitle ) {
 				document.title = this.originalDocumentTitle;
 			}
-
-			// Return focus to link that was originally clicked.
-			if ( this.link ) {
-				this.link.focus();
-			}
 		},
 
 		/**
@@ -225,6 +220,10 @@
 			Loader.saved     = null;
 			Loader.body.removeClass( 'customize-active full-overlay-active' ).removeClass( 'customize-loading' );
 			$( window ).off( 'beforeunload', Loader.beforeunload );
+			// Return focus to link that was originally clicked after visibility is restored.
+			if ( Loader.link ) {
+				Loader.link.focus();
+			}
 		},
 
 		/**
@@ -231,7 +230,7 @@
 		 * 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' );
 		},
 
 		/**