Index: wp-includes/class-wp-customize.php
===================================================================
--- wp-includes/class-wp-customize.php	(revision 20414)
+++ wp-includes/class-wp-customize.php	(working copy)
@@ -29,7 +29,6 @@
 		add_action( 'setup_theme',  array( $this, 'setup_theme' ) );
 		add_action( 'admin_init',   array( $this, 'admin_init' ) );
 		add_action( 'wp_loaded',    array( $this, 'wp_loaded' ) );
-		add_action( 'admin_footer', array( $this, 'admin_footer' ) );
 
 		add_action( 'customize_previewing',               array( $this, 'customize_previewing' ) );
 		add_action( 'customize_register',                 array( $this, 'register_controls' ) );
@@ -275,8 +274,6 @@
 		if ( isset( $_REQUEST['save_customize_controls'] ) )
 			$this->save();
 
-		wp_enqueue_script( 'customize-loader' );
-
 		if ( ( defined( 'DOING_AJAX' ) && DOING_AJAX ) )
 			return;
 
@@ -295,24 +292,6 @@
 	}
 
 	/**
-	 * Print the customize template.
-	 *
-	 * @since 3.4.0
-	 */
-	public function admin_footer() {
-		?>
-		<div id="customize-container" class="wp-full-overlay">
-			<input type="hidden" class="admin-url" value="<?php echo esc_url( admin_url( 'admin.php' ) ); ?>" />
-			<a href="#" class="close-full-overlay"><?php printf( __( '&larr; Return to %s' ), get_admin_page_title() ); ?></a>
-			<a href="#" class="collapse-sidebar button-secondary" title="<?php esc_attr_e('Collapse Sidebar'); ?>">
-				<span class="collapse-sidebar-label"><?php _e('Collapse'); ?></span>
-				<span class="collapse-sidebar-arrow"></span>
-			</a>
-		</div>
-		<?php
-	}
-
-	/**
 	 * Switch the theme and trigger the save action of each setting.
 	 *
 	 * @since 3.4.0
Index: wp-includes/js/customize-loader.dev.js
===================================================================
--- wp-includes/js/customize-loader.dev.js	(revision 20414)
+++ wp-includes/js/customize-loader.dev.js	(working copy)
@@ -41,7 +41,7 @@
 	$( function() {
 		Loader.initialize();
 
-		$('#current-theme, #availablethemes').on( 'click', '.load-customize', function( event ) {
+		$('#wpbody').on( 'click', '.load-customize', function( event ) {
 			var load = $(this);
 
 			event.preventDefault();
Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 20414)
+++ wp-includes/theme.php	(working copy)
@@ -1563,9 +1563,16 @@
 	}
 }
 
-function wp_customize_load() {
+/**
+ * Initialize the WP_Customize class.
+ *
+ * Fires when ?customize=on.
+ *
+ * @since 3.4.0
+ */
+function wp_customize_init() {
 	// Load on themes.php or ?customize=on
-	if ( ! ( isset( $_REQUEST['customize'] ) && 'on' == $_REQUEST['customize'] ) && 'themes.php' != $GLOBALS['pagenow'] )
+	if ( ! ( isset( $_REQUEST['customize'] ) && 'on' == $_REQUEST['customize'] ) )
 		return;
 
 	require( ABSPATH . WPINC . '/class-wp-customize.php' );
@@ -1573,4 +1580,33 @@
 	// @todo Dependency injection instead
 	$GLOBALS['customize'] = new WP_Customize;
 }
-add_action( 'plugins_loaded', 'wp_customize_load' );
+add_action( 'plugins_loaded', 'wp_customize_init' );
+
+/**
+ * Includes the loading scripts for the theme customizer and
+ * adds the action to print the customize container template.
+ *
+ * @since 3.4.0
+ */
+function wp_customize_loader() {
+	wp_enqueue_script( 'customize-loader' );
+	add_action( 'admin_footer', '_wp_customize_loader_template' );
+}
+
+/**
+ * Print the customize container template.
+ *
+ * @since 3.4.0
+ */
+function _wp_customize_loader_template() {
+	?>
+	<div id="customize-container" class="wp-full-overlay">
+		<input type="hidden" class="admin-url" value="<?php echo esc_url( admin_url( 'admin.php' ) ); ?>" />
+		<a href="#" class="close-full-overlay"><?php printf( __( '&larr; Return to %s' ), get_admin_page_title() ); ?></a>
+		<a href="#" class="collapse-sidebar button-secondary" title="<?php esc_attr_e('Collapse Sidebar'); ?>">
+			<span class="collapse-sidebar-label"><?php _e('Collapse'); ?></span>
+			<span class="collapse-sidebar-arrow"></span>
+		</a>
+	</div>
+	<?php
+}
\ No newline at end of file
Index: wp-includes/general-template.php
===================================================================
--- wp-includes/general-template.php	(revision 20414)
+++ wp-includes/general-template.php	(working copy)
@@ -2108,7 +2108,7 @@
  * Enqueues the default ThickBox js and css.
  *
  * If any of the settings need to be changed, this can be done with another js
- * file similar to media-upload.js and theme-preview.js. That file should
+ * file similar to media-upload.js. That file should
  * require array('thickbox') to ensure it is loaded after.
  *
  * @since 2.5.0
Index: wp-includes/script-loader.php
===================================================================
--- wp-includes/script-loader.php	(revision 20414)
+++ wp-includes/script-loader.php	(working copy)
@@ -364,8 +364,9 @@
 
 		$scripts->add( 'admin-widgets', "/wp-admin/js/widgets$suffix.js", array( 'jquery-ui-sortable', 'jquery-ui-draggable', 'jquery-ui-droppable' ), false, 1 );
 
-		$scripts->add( 'theme', "/wp-admin/js/theme$suffix.js", array( 'thickbox' ), false, 1 );
+		$scripts->add( 'theme', "/wp-admin/js/theme$suffix.js", array( 'jquery' ), false, 1 );
 
+		// @todo: Core no longer uses theme-preview.js. Remove?
 		$scripts->add( 'theme-preview', "/wp-admin/js/theme-preview$suffix.js", array( 'thickbox', 'jquery' ), false, 1 );
 
 		$scripts->add( 'inline-edit-post', "/wp-admin/js/inline-edit-post$suffix.js", array( 'jquery', 'suggest' ), false, 1 );
Index: wp-admin/includes/class-wp-upgrader.php
===================================================================
--- wp-admin/includes/class-wp-upgrader.php	(revision 20414)
+++ wp-admin/includes/class-wp-upgrader.php	(working copy)
@@ -711,7 +711,7 @@
 		$this->strings['process_success_specific'] = $this->strings['parent_theme_install_success'];//, $api->name, $api->version);
 
 		$this->skin->feedback('parent_theme_prepare_install', $api->name, $api->version);
-		
+
 		add_filter('install_theme_complete_actions', '__return_false', 999); // Don't show any actions after installing the theme.
 
 		// Install the parent theme
@@ -735,7 +735,7 @@
 
 		return $install_result;
 	}
-	
+
 	function hide_activate_preview_actions($actions) {
 		unset($actions['activate'], $actions['preview']);
 		return $actions;
@@ -1494,20 +1494,34 @@
 			return;
 
 		$theme_info = $this->upgrader->theme_info();
-		if ( empty($theme_info) )
+		if ( empty( $theme_info ) )
 			return;
-		$name = $theme_info->display('Name');
+
+		$name       = $theme_info->display('Name');
 		$stylesheet = $this->upgrader->result['destination_name'];
-		$template = $theme_info->get_template();
+		$template   = $theme_info->get_template();
 
-		$preview_link = htmlspecialchars( add_query_arg( array('preview' => 1, 'template' => $template, 'stylesheet' => $stylesheet, 'preview_iframe' => 1, 'TB_iframe' => 'true' ), trailingslashit(esc_url(get_option('home'))) ) );
-		$activate_link = wp_nonce_url("themes.php?action=activate&amp;template=" . urlencode($template) . "&amp;stylesheet=" . urlencode($stylesheet), 'switch-theme_' . $template);
+		$preview_link = add_query_arg( array(
+			'preview'    => 1,
+			'template'   => $template,
+			'stylesheet' => $stylesheet,
+		), trailingslashit( get_home_url() ) );
 
-		$install_actions = array(
-			'preview' => '<a href="' . $preview_link . '" class="thickbox thickbox-preview" title="' . esc_attr(sprintf(__('Preview &#8220;%s&#8221;'), $name)) . '">' . __('Preview') . '</a>',
-			'activate' => '<a href="' . $activate_link . '" class="activatelink" title="' . esc_attr( sprintf( __('Activate &#8220;%s&#8221;'), $name ) ) . '">' . __('Activate') . '</a>'
-							);
+		$customize_attributes = 'title="' . esc_attr( sprintf( __( 'Customize &#8220;%s&#8221;' ), $name ) ) . '"
+			. data-customize-template="' . esc_attr( $template ) . '" data-customize-stylesheet="' . esc_attr( $stylesheet ) . '"';
 
+		$activate_link = add_query_arg( array(
+			'action'     => 'activate',
+			'template'   => $template,
+			'stylesheet' => $stylesheet,
+		), admin_url('themes.php') );
+		$activate_link = wp_nonce_url( $activate_link, 'switch-theme_' . $template );
+
+		$install_actions = array();
+		$install_actions['preview']  = '<a href="' . esc_url( $preview_link ) . '" class="hide-if-js" title="' . esc_attr(sprintf(__('Preview &#8220;%s&#8221;'), $name ) ) . '">' . __('Preview') . '</a>';
+		$install_actions['preview'] .= '<a href="#" class="hide-if-no-js load-customize" ' . $customize_attributes . '>' . __('Customize') . '</a>';
+		$install_actions['activate'] = '<a href="' . esc_url( $activate_link ) . '" class="activatelink" title="' . esc_attr( sprintf( __('Activate &#8220;%s&#8221;'), $name ) ) . '">' . __('Activate') . '</a>';
+
 		if ( is_network_admin() && current_user_can( 'manage_network_themes' ) )
 			$install_actions['network_enable'] = '<a href="' . esc_url( wp_nonce_url( 'themes.php?action=enable&amp;theme=' . $template, 'enable-theme_' . $template ) ) . '" title="' . esc_attr__( 'Enable this theme for all sites in this network' ) . '" target="_parent">' . __( 'Network Enable' ) . '</a>';
 
@@ -1550,17 +1564,31 @@
 
 		$update_actions = array();
 		if ( ! empty( $this->upgrader->result['destination_name'] ) && $theme_info = $this->upgrader->theme_info() ) {
-			$name = $theme_info->display('Name');
+			$name       = $theme_info->display('Name');
 			$stylesheet = $this->upgrader->result['destination_name'];
-			$template = $theme_info->get_template();
+			$template   = $theme_info->get_template();
 
-			$preview_link = htmlspecialchars( add_query_arg( array('preview' => 1, 'template' => $template, 'stylesheet' => $stylesheet, 'TB_iframe' => 'true' ), trailingslashit(esc_url(get_option('home'))) ) );
-			$activate_link = wp_nonce_url("themes.php?action=activate&amp;template=" . urlencode($template) . "&amp;stylesheet=" . urlencode($stylesheet), 'switch-theme_' . $template);
+			$preview_link = add_query_arg( array(
+				'preview'    => 1,
+				'template'   => $template,
+				'stylesheet' => $stylesheet,
+			), trailingslashit( get_home_url() ) );
 
-			$update_actions['preview'] = '<a href="' . $preview_link . '" class="thickbox thickbox-preview" title="' . esc_attr(sprintf(__('Preview &#8220;%s&#8221;'), $name)) . '">' . __('Preview') . '</a>';
-			$update_actions['activate'] = '<a href="' . $activate_link . '" class="activatelink" title="' . esc_attr( sprintf( __('Activate &#8220;%s&#8221;'), $name ) ) . '">' . __('Activate') . '</a>';
+			$customize_attributes = 'title="' . esc_attr( sprintf( __( 'Customize &#8220;%s&#8221;' ), $name ) ) . '"
+				. data-customize-template="' . esc_attr( $template ) . '" data-customize-stylesheet="' . esc_attr( $stylesheet ) . '"';
 
-			if ( ( ! $this->result || is_wp_error($this->result) ) || $stylesheet == get_stylesheet() )
+			$activate_link = add_query_arg( array(
+				'action'     => 'activate',
+				'template'   => $template,
+				'stylesheet' => $stylesheet,
+			), admin_url('themes.php') );
+			$activate_link = wp_nonce_url( $activate_link, 'switch-theme_' . $template );
+
+			$update_actions['preview']  = '<a href="' . esc_url( $preview_link ) . '" class="hide-if-js" title="' . esc_attr(sprintf(__('Preview &#8220;%s&#8221;'), $name ) ) . '">' . __('Preview') . '</a>';
+			$update_actions['preview'] .= '<a href="#" class="hide-if-no-js load-customize" ' . $customize_attributes . '>' . __('Customize') . '</a>';
+			$update_actions['activate'] = '<a href="' . esc_url( $activate_link ) . '" class="activatelink" title="' . esc_attr( sprintf( __('Activate &#8220;%s&#8221;'), $name ) ) . '">' . __('Activate') . '</a>';
+
+			if ( ( ! $this->result || is_wp_error( $this->result ) ) || $stylesheet == get_stylesheet() )
 				unset($update_actions['preview'], $update_actions['activate']);
 		}
 
Index: wp-admin/update.php
===================================================================
--- wp-admin/update.php	(revision 20414)
+++ wp-admin/update.php	(working copy)
@@ -152,8 +152,8 @@
 
 		check_admin_referer('upgrade-theme_' . $theme);
 
-		add_thickbox();
-		wp_enqueue_script('theme-preview');
+		wp_customize_loader();
+
 		$title = __('Update Theme');
 		$parent_file = 'themes.php';
 		$submenu_file = 'themes.php';
@@ -204,8 +204,8 @@
 		if ( is_wp_error($api) )
 	 		wp_die($api);
 
-		add_thickbox();
-		wp_enqueue_script('theme-preview');
+	 	wp_customize_loader();
+
 		$title = __('Install Themes');
 		$parent_file = 'themes.php';
 		$submenu_file = 'themes.php';
@@ -230,11 +230,12 @@
 
 		$file_upload = new File_Upload_Upgrader('themezip', 'package');
 
+		wp_customize_loader();
+
 		$title = __('Upload Theme');
 		$parent_file = 'themes.php';
 		$submenu_file = 'theme-install.php';
-		add_thickbox();
-		wp_enqueue_script('theme-preview');
+
 		require_once(ABSPATH . 'wp-admin/admin-header.php');
 
 		$title = sprintf( __('Installing Theme from uploaded file: %s'), basename( $file_upload->filename ) );
Index: wp-admin/theme-install.php
===================================================================
--- wp-admin/theme-install.php	(revision 20414)
+++ wp-admin/theme-install.php	(working copy)
@@ -30,9 +30,6 @@
 	$submenu_file = 'themes.php';
 
 wp_enqueue_script( 'theme-install' );
-
-add_thickbox();
-wp_enqueue_script( 'theme-preview' );
 wp_enqueue_script( 'theme' );
 
 $body_id = $tab;
Index: wp-admin/themes.php
===================================================================
--- wp-admin/themes.php	(revision 20414)
+++ wp-admin/themes.php	(working copy)
@@ -66,9 +66,8 @@
 	'<p>' . __('<a href="http://wordpress.org/support/" target="_blank">Support Forums</a>') . '</p>'
 );
 
-add_thickbox();
-wp_enqueue_script( 'theme-preview' );
 wp_enqueue_script( 'theme' );
+wp_customize_loader();
 
 endif;