Do you want the Disqus comment system WordPress plugin to match your Twenty Fifteen WordPress theme? Use the following CSS:
#disqus_thread {
background: #fff;
padding: 3.8461% 7.6923%;
box-shadow: 0 0 1px rgba(0, 0, 0, 0.15);
width: 83.5882%;
margin: 2em auto;
}