I don't think you can do this. At least not easily. According to Adobe, TLF doesn't support CSS style declarations:
http://help.adobe.com/en_US/flex/using/WS02f7d8d4857b1677-165a04e1126951a2d98-7ff2.html
However, you might find 3rd party implementations that work:
https://dev.c-ware.de/confluence/display/PUBLIC/Flex4+Text-Flow+and+CSS-Styles
(I haven't tried it myself).