Sorry Lovere, that's not quite correct. The defaul_template directory does not contain the default template. It contains a guide for creating a custom template without tables.
Maci is using the template_default files to make changes to the look of his site.
This is an excellent move as the table-based Classic template will not be supported in the near future.
The template_default files are to be used as the foundation for creating your own "house." However, any change you make to something under template_default will be overridden in the next release UNLESS you use the override system.
The override system (also called custom template) has been put in place to help those who want the site to look a certain way even after update.
The devs designed most of the core files so that they look to see if any custom template work has been done before using the default.
The wiki, faq, and the forum have lots of help on the override system (custom template).
The override is not in place for the admin files as yet but, anytime you change a file under the mycart/includes/ directory, you should try to save it to an override directory. In maci's case the public_html/includes/templates/maci/css/stylesheet.css will keep his stylesheet intact even with a new release. That's the best and correct approach in maci's case.