The Advanced WordPress Theme Editor plugin will allow you to edit your theme directly in your blog admin section but with a better experience since it will format and highlight your source code with nice colours.
The plugin is compatible with WP 2.8 and less and is based on the Edit Area script that supports at least PHP, CSS, JS and many more file types.

Advanced WordPress Theme Editor

This plugin will improve your experience when editing yout wp themes since it will format and highlight your source code with nice colours.

Sergey Biryukov

WordPress Multisite (3.0+) already comes with theme and plugin editor enabled by default. This plugin is available for reference only.
Allows to enable theme and plugin editor for site administrator in WordPress MU. 
Requires Site Admin capabilities, not just blog administrator.
Based on the <a href="http://www.clickonf5.org/wordpress-mu/enable-theme-plugin-editor-wordpress-mu/5790" rel="nofollow ugc">non-plugin solution</a> by Sanjeev Mishra.

Enable Theme and Plugin Editor (WPMU)

Allows to enable theme and plugin editor for site administrator in WordPress MU.

Advanced WordPress Theme Editor vs Enable Theme and Plugin Editor (WPMU)