Update shame of theme.json file

In accordance with
https://github.com/WordPress/gutenberg/pull/28110

the theme.json file has been updated to reflect the new expected shape.

spearhead/experimental-theme.json

@@ -1,12 +1,14 @@
 
 {
-	"global": {
-        "styles": {
+    "styles": {
+		"root": {
             "color": {
                 "link": "var( --global--color-primary )"
             }
-        },
-		"settings": {
+		}
+	},
+	"settings": {
+		"defaults": {
 			"border": {
         		"customRadius": true
 			}