This morning I got this Idea. Hopefully this is the place to share it.
Why WordPress don't use the FTP user for the Theme Editor?. Using FTP user like on the installation of plugins, you will solve the problem of permissions for editing files with theme editor.
I mean you have to put files and directory to 777 to edit the files with the theme editor.
Also you can put your FTP settings on wp-config.php so you will no have to put the FTP password and user every time you save the file.
The idea came to me when I was thinking on a client that wants to change de "about us", with some cool HTML code and he knows a bit of HTML.