document.addEventListener('DOMContentLoaded', init);

function init(){
    var elem = document.getElementById('on');
    elem.addEventListener('click',funcon);
	var elem2 = document.getElementById('off');
    elem2.addEventListener('click',funcoff); 
}

function funcon(){
	chrome.tabs.insertCSS({
		code: "body{transition: opacity 0.2s ease-in; opacity: 1}"
	});
}

function funcoff(){
	chrome.tabs.insertCSS({
		code: "body{transition: opacity 0.2s ease-in; opacity: 0}"
	});
}