Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 19998)
+++ wp-includes/theme.php	(working copy)
@@ -475,7 +475,7 @@
 	global $wp_theme_directories;
 
 	if ( count($wp_theme_directories) <= 1 )
-		return '/themes';
+		return get_theme_root();
 
 	$theme_roots = get_site_transient( 'theme_roots' );
 	if ( false === $theme_roots ) {
@@ -547,22 +547,19 @@
  * @param string $directory Either the full filesystem path to a theme folder or a folder within WP_CONTENT_DIR
  * @return bool
  */
-function register_theme_directory( $directory) {
+function register_theme_directory( $directory ) {
 	global $wp_theme_directories;
 
-	/* If this folder does not exist, return and do not register */
-	if ( !file_exists( $directory ) )
-			/* Try prepending as the theme directory could be relative to the content directory */
-		$registered_directory = WP_CONTENT_DIR . '/' . $directory;
-	else
-		$registered_directory = $directory;
+	if ( ! file_exists( $directory ) ) {
+		// Try prepending as the theme directory could be relative to the content directory
+		$directory = WP_CONTENT_DIR . '/' . $directory;
+		// If this directory does not exist, return and do not register
+		if ( ! file_exists( $directory ) )
+			return false;
+	}
 
-	/* If this folder does not exist, return and do not register */
-	if ( !file_exists( $registered_directory ) )
-		return false;
+	$wp_theme_directories[] = $directory;
 
-	$wp_theme_directories[] = $registered_directory;
-
 	return true;
 }
 
@@ -660,14 +657,10 @@
  * @return string Theme path.
  */
 function get_theme_root( $stylesheet_or_template = false ) {
-	if ( $stylesheet_or_template ) {
-		if ( $theme_root = get_raw_theme_root($stylesheet_or_template) )
-			$theme_root = WP_CONTENT_DIR . $theme_root;
-		else
-			$theme_root = WP_CONTENT_DIR . '/themes';
-	} else {
+	if ( $stylesheet_or_template && $theme_root = get_raw_theme_root( $stylesheet_or_template ) )
+		$theme_root = $theme_root;
+	else
 		$theme_root = WP_CONTENT_DIR . '/themes';
-	}
 
 	return apply_filters( 'theme_root', $theme_root );
 }
@@ -685,7 +678,7 @@
 function get_theme_root_uri( $stylesheet_or_template = false ) {
 	if ( $stylesheet_or_template ) {
 		if ( $theme_root = get_raw_theme_root($stylesheet_or_template) )
-			$theme_root_uri = content_url( $theme_root );
+			$theme_root_uri = content_url( str_replace( WP_CONTENT_DIR, '', $theme_root ) );
 		else
 			$theme_root_uri = content_url( 'themes' );
 	} else {
@@ -696,10 +689,13 @@
 }
 
 /**
- * Get the raw theme root relative to the content directory with no filters applied.
+ * Get the raw theme root with no filters applied.
  *
  * @since 3.1.0
  *
+ * Before 3.4.0, this incorrectly returned a path relative to the content directory ("/themes") when
+ * only one theme directory was registered. Absolute paths are now always returned.
+ *
  * @param string $stylesheet_or_template The stylesheet or template name of the theme
  * @return string Theme root
  */
@@ -707,7 +703,7 @@
 	global $wp_theme_directories;
 
 	if ( count($wp_theme_directories) <= 1 )
-		return '/themes';
+		return WP_CONTENT_DIR . '/themes';
 
 	$theme_root = false;