The class for that part is "generalboxcontent", so change in your theme, styles.php, the background color for generalboxcontent.
It's possible that this affects other parts of Moodle.
A tip: use firefox, download the webdeveloper extention and use information/display ID and class details -> you can see then all the class-information of a page, so you'll know what to change.
(why am I writing this in English)
General developer forum
change colors
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