little confused on editing the basics of this theme

how exactly is this theme highly customizable? when editing the most basic part of the theme (the header) i can’t do it using the wordpress theme editor.

then i have to find default.css which is for some reason embedded in thematic/library/styles (not in the main stylesheet.css)

then i wanted to modify the branding #div within the header, however the only thing i see to edit is ‘<?php thematic_header() ?>’

so again i’m going through all folders on my local drive to find some type of header file to edit, and it’s not even called ‘thematic_header.php’, it’s ‘header-extensions.php’

what am i missing here? i must be approaching this the completely wrong way. thanks for any help.