Add primary dark to the JS for varia color annotations

varia/inc/color-annotations-preview.js

@@ -71,8 +71,10 @@ function changeColorLuminescence( hex, amount ) {
 
 			if( primary !== '' ){
 				var primaryHover = changeColorLuminescence( primary, 10 );
+				var primaryDark = changeColorLuminescence( primary, -10 );
 				cssVariables += '--wp--preset--color--primary: ' + primary + ';' +
 				'--wp--preset--color--primary-hover: ' + primaryHover + ';';
+				'--wp--preset--color--primary-dark: ' + primaryDark + ';';
 			}
 
 			if( secondary !== '' ){