CSS and system colors

On MS-Windows, and probably on other UIs, there are system colors
such as the default background color of a window (COLOR_WINDOW)
and the default foreground color of text (COLOR_WINDOWTEXT) (and
many more).

In the current version of CSS, the only way I can see to cause those
colors to come up on everyone's machines is to not define any colors
in a complete branch of the html tree right up to the HTML root
AND hope that the UA's default colors match those of the system.
Then that branch will use those system colors.

Should there be a more explicit way to get to those colors?