Unfortunately for you, your theme apparently does not use unique classes for that page, so changing the CSS for that page will affect all other pages.
Here is the CSS for that page. If you add it to the end of style.css, you will see what I mean:
`.posttop { width: 850px; background-size: 870px 325px; }
.postwrap { width: 850px; }
.postbottom { width: 870px; background-size: 870px 105px; margin: -30px 30px; }