How to make the Rounded corners of the client app in WindowedApplication?

In my application,showFlexChrome is set as true in WindowedApplication,and the *-app.xml is setted as follows

<systemChrome>none</systemChrome> 
<transparent>true</transparent> 
<visible>true</visible>

I know how to make the top border rounded corners with follow codes:

borderStyle="solid" cornerRadius="8" 

But I cannot find a method to make the bottom border of my app window rounded-corn开发者_运维问答er. Suggestions would be appreciated!

Regards

I've used a picture with rounded corners as a background image. Of course chrome was set to none and transparent to true.