Add support for a dark theme

[WardBrian]

Submission Checklist

• [x] Builds locally
• [x] New functions marked with <<{ since VERSION }>>
• [x] Declare copyright holder and open-source license: see below

Summary

Closes #738.

This adds a basic dark theme and cleans up some of our existing theme customizations

It still needs some color tweaks, especially for code blocks.

Copyright and Licensing

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By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC BY-ND 4.0 (https://creativecommons.org/licenses/by-nd/4.0/)

[mitzimorris]

FWIW, I used an online website to get the dark colors corresponding to primary color aquatic blue and lightest-beige, and it gave me this set:

/** SCSS DARK THEME PRIMARY COLORS */ 
 $primary-100:  #3e8ebc;
 $primary-200:  #599ac3;
 $primary-300:  #71a6cb;
 $primary-400:  #86b2d2;
 $primary-500:  #9bbfda;
 $primary-600:  #afcbe1;

 /** SCSS DARK THEME SURFACE COLORS */ 
 $surface-100:  #000000;
 $surface-200:  #1e1e1e;
 $surface-300:  #353535;
 $surface-400:  #4e4e4e;
 $surface-500:  #696969;
 $surface-600:  #858585;

 /** SCSS DARK THEME MIXED SURFACE COLORS */ 
 $surface-mixed-100:  #26465a;
 $surface-mixed-200:  #3d586b;
 $surface-mixed-300:  #546b7c;
 $surface-mixed-400:  #6b7f8e;
 $surface-mixed-500:  #8393a0;
 $surface-mixed-600:  #9ba8b2;

 /** EXAMPLES */ 
 color:  $surface-600; 
 background-color:  $primary-300;

[WardBrian]

I'm very happy if you or anyone else wants to tweak the colors here, I was mainly trying to get something working in Dark mode.

[mitzimorris]

getting this to work is awesome! happy to tweak colors.

[WardBrian]

Tweaked colors:

[mitzimorris]

text color looks good! not as harsh as before.

[WardBrian]

I'm sure we could continue tweaking forever, but I think it is over the "good enough" line now. Especially because the code blocks are actually readable!