I would like for the content of a Web page to follow the color scheme of the user's system. For example, if they are using a GTK- or QT-based Web browser, the background of the Web site would match the color of the background color in GTK or QT applications. Is there a way to set the color scheme of the Web page to match the user's system color scheme?
Only way to do that is by using deprecated CSS2 color value.
In your example background color would be as follows:
Here is a full specification from W3.org: http://www.w3.org/TR/css3-color/#css-system
If you write CSS3 or you want your application to be future-proof you shouldn't use these values. Also note that some of them might be available only on Windows.