This might save you some time if you’re building apps with Flex 4 Beta (Gumbo). In Spark Applications there is no backgroundAlpha property within you main MXML application file. To create a transparent Flex 4 background you’ll need to do the following:
1) Create a custom application skin MXML file.
2) Set the skinClass of your application MXML to the custom skin file: you can do this in the MXML for your app – eg. skinClass=”com.mycompany.myproject.skins.MyAppSkin”.
Search for the default ApplicationSkin.mxml in the Flex 4 SDK. Duplicate this file, rename it and place it within your project source. In the new skin code I just added alpha=”0″ to the backgroundRect declaration.
That’s it 🙂