This screen allows you to edit the custom skin (located under /skins/custom). 

You can edit any of the following files (provided it's writable by the server, e.g. CHMOD 766):

_main.php- This is the main template. It displays the blog.
_bloglist.php
skinshot.jpg
_feedback.php- This is the template that displays the feedback for a post
custom.css
_linkblog.php