There's a lot going on with your CSS... It'll take me some time to figure out what's going on, and right now I don't have the time.
You are repeating these selectors, and I don't understand why
.singular .entry-header,.singular .entry-content,.singular footer.entry-meta,.singular #comments-title {
width:68.9%;
margin:0 auto;
}
.one-column.singular .entry-header,.one-column.singular .entry-content,.one-column.singular footer.entry-meta,.one-column.singular #comments-title {
width:100%;
}
.singular .entry-header,.singular .entry-content,.singular footer.entry-meta,.singular #comments-title {
width:100%;
}
.singular .entry-header,.singular .entry-content,.singular footer.entry-meta,.singular #comments-title {
width:100%;
margin:0;
}
If you're repeating the above selectors, it's probably that others are being repeated as well.
Also, if you're making a few changes to the original CSS, you don't need to paste the whole CSS and select the "start from scratch" radio button (unless you want to use your themes markup as a "blank canvas").
Again, if you're making small changes to the original CSS (e. g. changing background colors) then I'd recommend you to reset the theme (delete entire CSS code in the CSS Editor and check the "add to current CSS" radio button) then, add ONLY those changes to the Editor (leaving the "add to current CSS" radio button checked). This article might tell you the benefits of why the CSS editor should only have your changes/additions to the CSS:
http://csswiz.wordpress.com/2009/10/15/if-you-have-the-wp-com-css-upgrade/