Get iframe by ID and append a CSS style to affect an element inside the iframe.

iframeAppendCSS

var iframe1 = document.getElementById('myiframeID');

iframe1.onload = function (){
  if(this.contentDocument) iframe1.doc = iframe1.contentDocument;
	else if(iframe.contentWindow) iframe1.doc = iframe1.contentWindow.document;
	
	var css = '.myelement{float:right;}',
		header = iframe1.doc.getElementsByTagName('head')[0],
		style = document.createElement('style');
		
	style.type = 'text/css';
	if (style.styleSheet){
	  style.styleSheet.cssText = css;
	} else {
	  style.appendChild(document.createTextNode(css));
	}

	header.appendChild(style);
}