This directory is meant for CSS files to include in all themes.
The currenly prefered way (as of Jul. 2007) is to @import all 
required CSS-es into global.css, and @import global.css into 
your skinn's CSS.

Given the common directory layout for themes, 
simply include in the theme's COMMON.CSS the line:

@import url(../../_css/global.css);

Cheers and happy styling.
