AddonSDK How to add a stylesheet

addonsdk.addstylesheet.js

const { browserWindows } = require("sdk/windows");

for(let w of browserWindows){
	AddStyleSheet("./myStyleSheet.css", viewFor(w) );
}


function AddStyleSheet(whatstylesheet,whatwindow){
	var attr = 'id="my-css" type="text/css" href="' + whatstylesheet + '"';
	var style = whatwindow.document.createProcessingInstruction('xml-stylesheet', attr);
	whatwindow.document.insertBefore(style, whatwindow.document.documentElement);
}