diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 3357532..6d0e4b2 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -635,15 +635,21 @@ function _theme_build_registry($theme, $base_theme, $theme_engine) {
  *   - 'info': The contents of the .info file.
  *   - 'stylesheets': A two dimensional array, using the first key for the
  *     'media' attribute (e.g. 'all'), the second for the name of the file
- *     (e.g. style.css). The value is a complete filepath
+ *     (e.g. style.css). The value is a complete filepath. Not set if no
+ *     stylesheets are defined in .info file
  *     (e.g. themes/bartik/style.css).
  *   - 'scripts': An associative array of JavaScripts, using the filename as key
- *     and the complete filepath as value.
+ *     and the complete filepath as value. Not set if no scripts are defined in
+ *     .info file.
+ *   - 'prefix': Defines theme's engine nameprefix. Unset if not defined in
+ *     .info file.
  *   - 'engine': The name of the theme engine.
- *   - 'base_theme': The name of the base theme.
+ *   - 'base_theme': If subtheme, the name of base theme. Otherwise not set.
  *   - 'base_themes': An ordered array of all the base themes. If the first item
- *     is NULL, a base theme is missing for this theme.
- *   - 'sub_themes': An unordered array of sub-themes of this theme.
+ *     is NULL, a base theme is missing for this theme. Not set if no base
+ *     themes are present.
+ *   - 'sub_themes': If theme has sub-themes, an unordered array of sub-themes.
+ *     Otherwise key is unset.
  */
 function list_themes($refresh = FALSE) {
   $list = &drupal_static(__FUNCTION__, array());