/***************************************
Initialize Variables
***************************************/
var tf:TextField = new TextField();
	tf.width = stage.stageWidth;
	tf.height = stage.stageHeight;
	addChild(tf);
	
var wordList:Array = new Array();

var textLoader:URLLoader = new URLLoader();
    textLoader.addEventListener(Event.COMPLETE, textLoaded);

var cssLoader:URLLoader = new URLLoader();
var css:StyleSheet = new StyleSheet();


/***************************************
Parse Text + Apply External CSS
***************************************/
function cssLoaded(e:Event):void{
	css.parseCSS(e.target.data);
	tf.styleSheet = css;
	
	for(var i:int = 0; i < wordList.length; i++){
		tf.htmlText += "<h4>" + wordList[i] + "</h4>";
	}
}
function textLoaded(e:Event):void{
	wordList = e.target.data.split("\n");
	cssLoader.load(new URLRequest("http://www.channelone.com/css/flash_block.css"));	
	cssLoader.addEventListener(Event.COMPLETE, cssLoaded);
}

textLoader.load(new URLRequest("http://www.channelone.com/fun/swf_band_name_txt.txt"));

Here is an intermediate example of how to load an external .txt file and apply an external .css

AS3: Load external .txt and .css

ActionScript 3