Back to Administration Screens.
The built-in Theme Editor allows you to view or change (see next paragraph) any code kept in a Theme Template or Stylesheet files. The particular file contents are displayed in the large text (or edit) box that dominates this Screen.
If a specific file is writable (see Changing File Permissions), changes can be made, and saved. If a file is not writable, the message You need to make this file writable before you can save your changes. See the Codex for more information. will be displayed. Of course, you will need to change the File Permissions before changes can be saved.
The name of the Theme file being edited is displayed at the top of the text box. If the full text of the file cannot be viewed, a scroll bar to the right of the text box is provided. Since Theme files are pure text, no images or pictures can be inserted into the text box.
From this pull-down box, select the Theme that needs to be edited and click the Select button on the right. Once the Select button is clicked that Themes Template and Styles files will be listed on the right side of the text box and the Stylesheet (style.css) file for that Theme will be placed in the text box.
Be very careful editing PHP files of your current theme. The editor does not make backup copies. If you introduce an error that crashes your site, you cannot use the editor to fix the problem. In such a case, use FTP to either upload a functional backup of the problem file or change the folder name of the current theme so WordPress is forced to use a different theme.