Projects - Flex Builder - Using External Style Sheets

.mxml code:
<?xml version="1.0" encoding="utf-8"?>
<mx:Application
xmlns:mx="http://www.adobe.com/2006/mxml"
width="150" height="140"
viewSourceURL="src/StylesExternal/index.html"

>
<mx:Style source="external.css" />

<mx:VBox styleName="solidBorder">

<mx:Button label="Submit"/>
</mx:VBox>

<mx:VBox styleName="solidBorderPaddedVertically">
<mx:Button label="Submit"/>
</mx:VBox>

</mx:Application>