This is IMHO totally counter productive.
There might be some remote cases where the nesting is better rendered with the left bar style, but for the most part you are breaking the intent of posters on SO who want the contrast provided by the light-yellow background.
This move does not fix any real problem. You should at least apply the common sense wisdom: if it aint broke, dont fix it.
Was there a vote on this change? Who makes this kind of decision?
PLEASE Change it back!