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.

Space X-Chimp

An easy to use, with intuitive interface, WordPress plugin that replaces the default Theme and Plugin Source Code Editor with an enhanced editor by a <a href="https://codemirror.net/" rel="nofollow ugc">CodeMirror library</a>. The WordPress default Theme Editor (in Appearance => Editor) and Plugin Editor (in Plugins => Editor) is great for doing some custom changes to your Themes or Plugins files, although it is rather limited. This is where the “Syntax Highlighter for Theme/Plugin Editor” can help. With an enhanced source code editor you can see a code with syntax highlighting, and with line numbering, so you can easily read your code and detect any errors.
This plugin give you finer control over editor, it can be configured on the plugin settings page. You can chose from 36 different color themes for your editor. If you want more options, then let us know and we will be happy to add them.
This plugin is just plug and play, no tedious configurations or hacks, just install, enable and start using your new enhanced Theme and Plugin Editor.
Features
<ul>
<li>Lightweight and fast</li>
<li>Secure code with using clear coding standards</li>
<li>Intuitive interface with many settings</li>
<li>Cross browser compatible (work smooth in any modern browser)</li>
<li>Compatible with all WordPress themes</li>
<li>RTL compatible (right to left)</li>
<li>Translation ready</li>
</ul>
Key features include
<ul>
<li>Code syntax highlighting</li>
<li>Huge selection of color schemes (36 themes)</li>
<li>Line numbering</li>
<li>Editor allow for Tab indentation</li>
<li>Live preview</li>
<li>And much, much more!</li>
</ul>
Supported languages:
<ul>
<li>XML</li>
<li>HTML</li>
<li>CSS</li>
<li>PHP</li>
<li>JavaScript (.js)</li>
<li>Markdown (.txt)</li>
</ul>
Translation
This plugin is ready for translation and has already been translated into several languages. But If your language is not available then you can make one. It is also possible that not all existing translations are up-to-date or correct, so you are welcome to make corrections. Many of plugin users would be delighted if you share your translation with the community. Thanks for your contribution!
<ul>
<li>English (default)</li>
<li>Russian (translation by <a href="https://www.instagram.com/milava_kiseleva/" rel="nofollow ugc">Milena Kiseleva</a>)</li>
<li>German (translation by Michael)</li>
<li>Spanish (translation by Ramiro Garcés and Patricio Toledo)</li>
<li>Dutch (translation by Peter Leenders)</li>
<li>French (translation by Jean-Michel, Theophil Bethel and Hervé Bouzin)</li>
</ul>
If you want to help translate this plugin, please visit the <a href="https://translate.wordpress.org/projects/wp-plugins/syntax-highlighter-for-wp-editor" rel="nofollow ugc">translation page</a>.
Minimum system requirements:
<ul>
<li><a href="https://wordpress.org">WordPress</a> version 4.9 or higher.</li>
<li><a href="https://secure.php.net" rel="nofollow ugc">PHP</a> version 5.6 or higher.</li>
<li><a href="https://www.mysql.com" rel="nofollow ugc">MySQL</a> version 5.0 or higher.</li>
</ul>
Recommended system requirements:
<ul>
<li><a href="https://wordpress.org">WordPress</a> version 5.0 or higher.</li>
<li><a href="https://secure.php.net" rel="nofollow ugc">PHP</a> version 7.0 or higher.</li>
<li><a href="https://www.mysql.com" rel="nofollow ugc">MySQL</a> version 5.6 or higher.</li>
</ul>
Contribution
Developing plugins is long and tedious work. If you benefit or enjoy this plugin please take the time to:
<ul>
<li><a href="https://www.spacexchimp.com/donate.html" rel="nofollow ugc">Donate</a> to support ongoing development. Your contribution would be greatly appreciated.</li>
<li><a href="https://wordpress.org/support/plugin/syntax-highlighter-for-wp-editor/reviews/#new-post">Rate and Review</a> this plugin.</li>
<li><a href="https://www.spacexchimp.com/contact.html" rel="nofollow ugc">Share with us</a> or view the <a href="https://github.com/ArthurGareginyan/syntax-highlighter-for-wp-editor" rel="nofollow ugc">GitHub Repo</a> if you have any ideas or suggestions to make this plugin better.</li>
</ul>
“Syntax Highlighter for Theme/Plugin Editor” is one of the own software projects of <a href="https://www.spacexchimp.com" rel="nofollow ugc">Space X-Chimp</a>.
License
This plugin is licensed under the <a href="http://www.gnu.org/licenses/gpl-3.0.html" rel="nofollow ugc">GNU General Public License, version 3 (GPLv3)</a> and is distributed free of charge. 
Commercial licensing (e.g. for projects that can’t use an open-source license) is available upon request.
Credits
<ul>
<li>The icon of this plugin is a copyrighted image created by the <a href="https://www.spacexchimp.com" rel="nofollow ugc">Space X-Chimp</a> team. (C) All rights reserved.</li>
<li>The banner of this plugin is a copyrighted image created by the <a href="https://www.spacexchimp.com" rel="nofollow ugc">Space X-Chimp</a> team. (C) All rights reserved.</li>
<li>Unless otherwise stated, all images are created by the <a href="https://www.spacexchimp.com" rel="nofollow ugc">Space X-Chimp</a> team and are copyrighted. (C) All rights reserved.</li>
<li><a href="https://codemirror.net/" rel="nofollow ugc">CodeMirror</a> is an open-source project shared under the <a href="https://codemirror.net/LICENSE" rel="nofollow ugc">MIT license</a>.</li>
<li><a href="http://getbootstrap.com" rel="nofollow ugc">Bootstrap</a> by Twitter, Inc. released under the <a href="https://github.com/twbs/bootstrap/blob/master/LICENSE" rel="nofollow ugc">MIT license</a>.</li>
<li><a href="https://github.com/vsn4ik/bootstrap-checkbox" rel="nofollow ugc">Bootstrap-checkbox</a> is a project of <a href="https://github.com/vsn4ik" rel="nofollow ugc">Vasily A.</a>, shared under the <a href="https://github.com/vsn4ik/bootstrap-checkbox/blob/master/LICENSE" rel="nofollow ugc">MIT license</a>.</li>
<li><a href="https://fontawesome.com" rel="nofollow ugc">Font Awesome</a> is an open source-project created by Dave Gandy. Font released under the <a href="http://scripts.sil.org/OFL" rel="nofollow ugc">SIL OFL 1.1 license</a>. Code released under the <a href="http://opensource.org/licenses/mit-license.html" rel="nofollow ugc">MIT License</a>.</li>
</ul>
Links
<ul>
<li><a href="https://www.spacexchimp.com" rel="nofollow ugc">Developer website</a></li>
<li><a href="https://github.com/ArthurGareginyan/syntax-highlighter-for-wp-editor" rel="nofollow ugc">Dedicated plugin page on GitHub</a></li>
<li><a href="https://wordpress.org/plugins/syntax-highlighter-for-wp-editor/">Dedicated plugin page on WordPress.org</a></li>
</ul>

Syntax Highlighter for Theme/Plugin Editor

Replaces the defaults WordPress Theme/Plugin Editor with an enhanced editor with syntax highlighting, line numbering, etc.

Advanced WordPress Theme Editor vs Syntax Highlighter for Theme/Plugin Editor