CSS Center Alignment

I’ll preface this by saying I’ve never developed with CSS but I have a reasonable understanding through context. This is a fresh site, only been up a few days and I’m working on a few tweaks to the theme. I have a post on the url and the featured media appears left aligned but I’d like it to either stretch to fill the confines of the post width or just be centred at the top of the post. I’ve been poking the theme CSS for a day and not had much luck figuring it out. If anyone can offer any assistance it would be most appreciated.

I believe this is the section I should be looking at.

/*  Featured media  ----------------------------------------- */


.featured-media a,
.featured-media img
.featured-media iframe { display: block; }

.featured-media iframe,
.featured-media object { border: none; margin: 0; }

.featured-media a { overflow: hidden; }

.featured-media a img { margin: 100 auto; }

.featured-media a:hover img { 
	transform: scale(1.1);
	opacity: 0.8;
}

.media-caption {
	font-size: 0.8em;
	line-height: 110%;
	font-weight: 700;
	color: #999;
	padding: 20px 0;
	margin: 0 10%;
	border-bottom: 1px solid #eee;
}

The page I need help with: [log in to see the link]