title: What's the deal with Halo and Spark in Flex 4?
body: <p>I used Flex 2/3 for a couple of years and have just come back to it, upgrading to Flex 4. I'm seeing some references to Halo and Spark themes but I never got into themes in my previous work so I'm a bit confused what's going on. Am I correct that Halo was the default theme, or is this all new?</p>
  
  <p>Also, I took a simple Flex 3 MXML and tried to make it work in Flex4:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;mx:Application
      xmlns:fx="http://ns.adobe.com/mxml/2009"
      xmlns:mx="library://ns.adobe.com/flex/mx"
      layout="absolute" borderStyle="none" borderColor="#000000" cornerRadius="0" 
      backgroundGradientAlphas="[1.0, 1.0]" backgroundGradientColors="[#000000, #FF0000]"
      width="800" height="600"
      frameRate="20"&gt;
  &lt;/mx:Application&gt;
  </code></pre>
  
  <p>But I get errors, like "<strong>Error: The style 'backgroundGradientColors' is only supported by type 'mx.core.Application' with the theme(s) 'halo'.</strong>"</p>
  
  <p>I tried a few things but each one introduced its own errors.</p>