diff --git a/wp-content/themes/twentyten/functions.php b/wp-content/themes/twentyten/functions.php
index d0257cffa..08446ee16 100644
--- a/wp-content/themes/twentyten/functions.php
+++ b/wp-content/themes/twentyten/functions.php
@@ -28,6 +28,9 @@ function twentyten_init() {
 
 	add_custom_background();
 
+	// This theme styles the visual editor with editor-style.css to match the theme style.
+	add_editor_style();
+
 	// This theme needs post thumbnails
 	add_theme_support( 'post-thumbnails' );
 
@@ -134,20 +137,6 @@ function twentyten_comment( $comment, $args, $depth ) {
 }
 endif;
 
-// Make the Visual Editor styles match the theme's styles
-if ( ! function_exists( 'twentyten_editor_style' ) ) :
-function twentyten_editor_style( $url ) {
-	if ( ! empty( $url ) )
-		$url .= ',';
-
-	// Change the path here if using sub-directory
-	$url .= trailingslashit( get_stylesheet_directory_uri() ) . 'editor-style.css';
-
-	return $url;
-}
-endif;
-add_filter( 'mce_css', 'twentyten_editor_style' );
-
 // Remove inline styles on gallery shortcode
 if ( ! function_exists( 'twentyten_remove_gallery_css' ) ) :
 function twentyten_remove_gallery_css() {
diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index 8b29dff66..741c66d36 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -1446,6 +1446,43 @@ body {
 <?php
 }
 
+/**
+ * Add callback for a custom TinyMCE editor stylesheet.
+ *
+ * The parameter $stylesheet is the name of the stylesheet, relative to
+ * the theme root. It is optional and defaults to 'editor-style.css'.
+ *
+ * @since 3.0.0
+ *
+ * @param callback $stylesheet Name of stylesheet relative to theme root.
+ */
+function add_editor_style( $stylesheet = 'editor-style.css' ) {
+	if ( isset( $GLOBALS['editor_style'] ) )
+		return;
+
+	add_theme_support( 'editor-style' );
+
+	if ( ! is_admin() )
+		return;
+
+	$GLOBALS['editor_style'] = $stylesheet;
+	add_filter( 'mce_css', '_editor_style_cb' );
+}
+
+/**
+ * Callback for custom editor stylesheet.
+ *
+ * @since 3.0.0
+ * @see add_editor_style()
+ * @access protected
+ */
+function _editor_style_cb( $url ) {
+	global $editor_style;
+	if ( ! empty( $url ) )
+		$url .= ',';
+	return $url . get_stylesheet_directory_uri() . "/$editor_style";
+}
+
 /**
  * Allows a theme to register its support of a certain feature
  *