CSS Overview

By default, sites are loaded with a full url to an un-editable set of style sheets (i.e. http://style1.orbisdesign.net/css/screen.css.

In addition, every site has a custom style sheet added (see control panel) css/custimport.css (loaded on every page), which allows overiding elements set within the base style sheets.
If desired, all the style sheets can be copied to the base CSS directory of the client, and the control panel can be changed to call that reletive style sheet base.
Any page can also load an indipendent style sheet by installing the display structure and then adding a reference to any specific style sheet (PageCSS)

Additional Notes

Admin CSS linkage takes precedence. If the style sheet linkage is absolute (i.e. http://style1.orbisdesign.net) then images and CSS are all relative to that URL and not from the website (i.e. not bonsai.orbisdesign.net/css).
If an application references a relative style as in an import, then the CSS directory of that website takes precedence since the style import comes after that initial linkage.
The page display custom import comes after the linkage, but before the application import so it works great as an overide but not if the import is done at the application level.