Need mx:textArea with transparent background and backgroundAlpha="0" won't work

I have a custom component that extends mx:TextArea, but when I try and apply backgroundAlpha="0" it says

The style 'backgroundAlpha' is only supported by type 'components.LabelEditor' with the theme(s) 'halo'.

I don't see anywhere specifying that it is not using the Halo theme, unless i开发者_运维百科t has decided to extend s:TextArea despite my not importing it.

How can I make this component accept the style instructions or achieve a transparent background some other way?

As per the documentation on themes:

The default theme for Flex 4 components is Spark. All controls in an application built with Flex 4 use the Spark theme. Even MX controls have a Spark look and feel in a Flex 4 application.

Also, as per the ASDocs for mx:TextArea, when using the Spark theme there is a style named 'contentBackgroundAlpha' that can be set achieve what you seem to want out of the mx:TextArea.