First of all: +1 from me! The sidebar on the left is great. It makes the main column more centered and thus it is significantly better for comfortable reading (on a wide screen). Seriously - a bigger left margin is a remedy to the former layout.
Though I have a pair of minor suggestions.
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Right sidebar & resizing the window ("responsive design")
Currently the right sidebar moves towards center when shrinking the window and then at some point - woop! it just dissapears. I see, this is kind of "responsive design". But is it possible to make it the other way: say if I shrink the window width, it would be better if the right sidebar at some moment starts to just 'clip' under the border. I mean it will just be clipping by the window border more and more when shrinking the window. And then, when it is away, the main column starts toshrink 'adaptively'.
Maybe it is hard to implement, but I beleive the users who need smaller views will much appreciate that.And there will be less rearrangement happening on the screen when resizing the window.
Also, the gap between the main column and the right sidebar is still too small. Please add ~ 15 pixels more spacing.
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Maybe add something useful to the new sidebar?
Consider adding e.g. "My profile" and "Other SE sites" to the left sidebar - anyway there is a lot of space.