[css-color-4] Add the buttonborder, mark, and marktext system colors, per WG resolution. w3c#4924

Author: tabatkins

css-color-4/Overview.bs

@@ -919,6 +919,8 @@ System Colors</h3>
 		<dd><span class="swatch" style="--color: buttonface"></span>&nbsp;The face background color for push buttons.
 		<dt><dfn>ButtonText</dfn>
 		<dd><span class="swatch" style="--color: buttontext"></span>&nbsp;Text on push buttons.
+		<dt><dfn>ButtonBorder</dfn>
+		<dd><span class="swatch" style="--color: buttonborder"><span>&nbsp;The base border color for push buttons.
 
 		<dt><dfn>Field</dfn>
 		<dd><span class="swatch" style="--color: field"></span>&nbsp;Background of input fields.
@@ -930,6 +932,13 @@ System Colors</h3>
 		<dt><dfn>HighlightText</dfn>
 		<dd><span class="swatch" style="--color: highlighttext"></span>&nbsp;Text of item(s) selected in a control.
 
+		<dt><dfn>Mark</dfn>
+		<dd><span class="swatch" style="--color: mark"></span>&nbsp;Background of text that has been specially marked
+			(such as by the HTML <{mark}> element).
+		<dt><dfn>MarkText</dfn>
+		<dd><span class="swatch" style="--color: marktext"></span>&nbsp;Text that has been specially marked
+			(such as by the HTML <{mark}> element).
+
 		<dt><dfn>GrayText</dfn>
 		<dd><span class="swatch" style="--color: graytext"></span>&nbsp;Disabled text.
 			(Often, but not necessarily, gray.)