Bump on this one, I'd like color-coding, line-numbers and keyboard-tab-capture behavior in the theme editor (Admin > Themes > Theme Editor). With my site being on a non-local host now, theme tweaks are more important than ever from the admin section.. and it's a pain to try and find your place again and again after making an edit midway down the css sheet or to try and use the browser's 'find' feature to locate a specific bit of text in the code being edited.
My greatest pet peeve? Trying to use Tab to insert a 5-7 space indentation in the code, and finding my browser focus has been sent to the next element in the page (classic keyboard shortcut in a browser.)
Still.. Nice being able to edit straight in the browser! Ditto on that plugin/hack thing.. anyone know of any?