[Wordpress issuses] mikeosbornphoto on "Change background colour for comments"

Hello there,
I recently changed to a lighter colour scheme for my Nishita-themed blog. One snag - contributors' comments are now framed in a dark box, which makes them hard to read and is ugly to the eye:

http://mikeosbornphoto.com/2013/04/07/white-city-portrait/#comments

I'd like all the comments to appear on a plain white background, like Chris Kemp's in the above example.

I understand with my customised upgrade I need to alter some code in the CSS. Would anyone be able to tell me what this is?

Many thanks for your help,
Mike

The blog I need help with is mikeosbornphoto.com.

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