Add test for custom default colors

Author: JayPanoz

backstop.json

@@ -78,6 +78,13 @@
       "selectors": ["document"],
       "misMatchThreshold" : 0.1
     },
+    {
+      "label": "Default custom colors pref",
+      "url": "http://localhost:8000/tests/default-custom-colors.html",
+      "delay": 0,
+      "selectors": ["document"],
+      "misMatchThreshold" : 0.1
+    },
     {
       "label": "Sepia mode pref",
       "url": "http://localhost:8000/tests/sepia.html",