[issue23951] Update devguide style to use a similar theme as Docs

[Zachary Ware]

Zachary Ware added the comment:

One benefit to having a different theme for the devguide is that it makes clear that "you're not in the standard library docs anymore".  I agree that that screenshot looks quite a bit better, though.

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nosy: +zach.ware

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<http://bugs.python.org/issue23951>
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