GetSimple provides you a simple way to quickly edit any theme you currently have installed on your system. It does NOT have to be enabled to be able to edit it.

Simply goto Theme → Edit Theme and choose the file you want to edit. Be warned that there are no backups taken before or after these edits. If you make a mistake, you will need to fix it yourself by either downloading the theme again (if it's available) or attempting to fix it thru code.