Google's Custom Search Engine styles have been changed so the ad-blocks at the top/right of the search results have a different background colour. Ref: http://googlecustomsearch.blogspot.ie/2012/06/ads-background-colors-in-custom-search.html
It looks like we cannot control this colour - it's been chosen to "contrast in a visually appealing way with the default [or user-selected] background color".
In at least one of my blogs, though, the contrast between the ad-background colour and the outer background color selected in blogger isn't quite so "appealing". :-(
I'm still considering whether to change my outer background, or to experiment with different CSE custom styles to see if one is a better fit. Luckily most of my in-post features have been set up with CSS rules, so I can change them easily by adding a new CSS rule to my template.
