I am having the same problem. I like the 1200 width for the portfolio posts, though, but my first page is information so it becomes hard to read. So the customize appearance does not help me.
I was having the same problem, on posts thumbs, try to remove the lines:
.article-helper {
background: #fff;
-webkit-box-shadow: 0 0 13px rgba(0, 0, 0, .07);
-moz-box-shadow: 0 0 13px rgba(0, 0, 0, .07);
-ms-box-shadow: 0 0 13px rgba(0, 0, 0, .07);
box-shadow: 0 0 13px rgba(0, 0, 0, .07);
