[css-color-4] Add dfn for 'system color pairings'.

Author: tabatkins

css-color-4/Overview.bs

@@ -951,7 +951,7 @@ System Colors</h3>
 	For systems that do not have a particular system UI concept,
 	the specified value should be mapped to
 	the most closely related system color value that exists.
-	The following pairings are expected to form legible background-foreground colors:
+	The following <dfn export>system color pairings</dfn> are expected to form legible background-foreground colors:
 
 	<ul id=system-color-pairs>
 		* ''Canvas'' background with ''CanvasText'', ''LinkText'', ''VisitedText'', ''ActiveText'' foreground.