refactor: simplify ContributionPrompt and remove customCSS

[levithomason]

This is a breakout of ContributionPrompt cleanup form #16.

We had inline styles and an extra div applied to each instance of the contribution prompt. This just removes those styles, removes the docs/src/utils/customCSS file, and cleans up / simplifies the prompt.

It largely works and looks the same:

[codecov]

Codecov Report

Merging #68 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #68   +/-   ##
=======================================
  Coverage   85.92%   85.92%           
=======================================
  Files          74       74           
  Lines        1101     1101           
  Branches      216      216           
=======================================
  Hits          946      946           
  Misses        149      149           
  Partials        6        6

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