Anyone who’s used WordPressMu for a while has probably noticed that the standard WordPress theme editor (and the plug-in editor, for that matter) isn’t available in the administration area. There are some very valid security reasons for disabling the theme and plug-in editors by default, but, for those of us who keep pretty tight reigns on the admin area, the disadvantages of not being able to edit themes from within WordPress outweigh the security advantages of having the editor disabled.
For those people, there is a plug-in available that re-enables both editors. Simply download the plug-in, unzip it and upload the “enable-theme-and-plugin-editor.php” file to your mu-plugins directory. Once you’ve done that, the theme editor and plug-in editor will both be available from the admin menu again.