Select more accessible default colors

Fix mdn/content#15621

Author: dawei-wang

learn/cascade/all.html

@@ -11,8 +11,8 @@
 
     <style class="editable">
 blockquote {
-    background-color: red;
-    border: 2px solid green;
+    background-color: orange;
+    border: 2px solid blue;
 }
 
 .fix-this {
@@ -59,4 +59,4 @@
 </body>
 <script src="../playable.js"></script>
 
-</html>
+</html>