User-Defined System Colors
As of Microsoft Internet Explorer 4.0, system colors may be specified in Web pages in addition to the named colors. Unlike the other named colors however, user-defined system colors have no numeric RGB equivalent because the exact color is not known until the Web page is viewed on the user's system. Users can choose their own system color scheme from the Windows Control Panel.
The following image shows typical Windows 7 system colors.
Click the link below to see an active table of your current system colors.