CSS2 enables to use system colors (like ButtonText) instead of hard-coded colors (like black).
To see all usable combinations, display css2-colors-preview.html at your browser.
To include/modify pre-created css classes, look at the other file, css2-colors.css.
Note that these colors are officially deprecated in CSS3, but still supported in all major browsers. The reason for deprecation was to replace it by CSS3 appearance style which still lacks browsers support and even the standard needs to amend a lot of features here, while CSS2 colors is reliable cross-browser feature.
Enjoy these files published under WTFPL2 licence.