Rainfall: Add duotone to featured image

rainfall/theme.json

@@ -95,6 +95,11 @@
 					"textTransform": "uppercase"
 				}
 			},
+			"core/post-featured-image": {
+				"filter": {
+					"duotone": "var(--wp--preset--duotone--grayscale)"
+				}
+			},
 			"core/post-navigation-link": {
 				"typography": {
 					"textTransform": "uppercase"