[demouser]

Я так сделал:

function setStyle(style:Object, name:String = ''):void {
  if(!name) {
    // stylesheet can't set styles to text which is not wrapped by tags. Turn off stylesheet, set textformat and set stylesheet back.
    var styleSheet:StyleSheet = m_field.styleSheet;
    m_field.styleSheet = null;
    var tf:TextFormat = styleSheet.transform(style);
    m_field.defaultTextFormat = tf;
    m_field.setTextFormat(tf);
    m_field.styleSheet = styleSheet;
  }
  else m_field.styleSheet.setStyle(name, style);
}