Styles for dark mode

[AakashGfude]

Corrected styles for

• [x] math block.
• [x] note
• [x] admonitions
• [x] margins
• [x] overall background color
• [x] code block

A screenshot for the math block:

fixes https://github.com/QuantEcon/quantecon-book-theme/issues/214

fixes https://github.com/QuantEcon/quantecon-book-theme/issues/216

[codecov[bot]]

Codecov Report

Merging #215 (b6d1d67) into master (69418e6) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #215   +/-   ##
=======================================
  Coverage   71.12%   71.12%           
=======================================
  Files           2        2           
  Lines         239      239           
=======================================
  Hits          170      170           
  Misses         69       69           

Flag	Coverage Δ
pytests	71.12% <ø> (ø)

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[AakashGfude]

@mmcky some netlify rules are throwing errors, but that's not becuase of this. So, I will go ahead and merge.