This screen is accessed from the back-end Joomla! administrator panel. It is used to edit the source code of template files.
Navigate to the Template Manager: Customise Template screen. Click on the name of the template you want to edit. Then click the link to the template file that you want to edit.
This screen is where the source code of template files is edited. It provides a plain text interface to edit the template files. HTML and PHP programming syntax is highlighted to make the source code files easier to read.
At the top right you will see the toolbar: