Refactored theme.json

Refactored the experimental-theme.json to fit the structure for the
change as of Gutenberg 9.9

seedlet/experimental-theme.json

@@ -1,6 +1,6 @@
 {
-	"global": {
-		"styles": {
+	"styles": {
+		"root": {
 			"color": {
 				"link": "var( --global--color-primary )"
 			}