Merge pull request #3106 from Automattic/fix/3093

Add a theme.json file so that we can support border radius in Spearhead

spearhead/experimental-theme.json

@@ -0,0 +1,10 @@
+
+{
+	"global": {
+		"settings": {
+			"border": {
+        		"customRadius": true
+			}
+		}
+	}
+}