CSSStyleDeclaration at run time is not working

Question

My Code:

var TextStyleDeclaration:CSSStyleDeclaration=new CSSStyleDeclaration(".ImageElement");
TextStyleDeclaration.defaultFactory=function():void
{    
    leftMargin=50;
}
StyleManager.setStyleDeclaration(".ImageElement",TextStyleDeclaration,true);
var page_rich_text:spark.components.RichText=new spark.components.RichText();
page_rich_text.styleName="ImageElement";