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

[STINNER Victor]

STINNER Victor added the comment:

I pushed your new style, thanks. I didn't see any malware in the new Javascript nor new HTML :-)

I keep the issue open until the online doc is updated.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue23951>
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