You can use getComputedStyle().
getComputedStyle()
gives the final used values of all the CSS properties of an element.
var element = document.getElementById('sidebar'),
style = window.getComputedStyle(element),
display = style.getPropertyValue('display');
var element = document.getElementById('video_container'),
style = window.getComputedStyle(element),
bg = style.getPropertyValue('background-color');