While I am not a fully-fledged designer I would experiment with changing the background colour to an off-white, something like: #EFEFEF - you'll notice if you do that then things are easier to read immediately because content is being separated into its respected sections.

The white you have around each submission is fine, changing the background colour seems to make it easier (at least for me) to read the site. There are probably other ways, but this is the first immediate thing I can recommend.