Stewart: removing no needed definitions in thheme.json (#5938)

stewart/theme.json

@@ -285,8 +285,6 @@
 			},
 			"core/separator": {
 				"border": {
-					"color": "currentColor",
-					"style": "solid",
 					"width": "0 0 1px 0"
 				}
 			},