Dark Mode (#168)

* Implement toggle button

* Dark mode mixins

* Dark mode color design

* Disable dark mode for prints

* Initial dark mode design

* Fix light mode mixin

* Disable dark mode on print style

* Move dark mode color scheme below the default style to allow referencing

* Fix dropdown

* Fix device-based dark mode

* Dark mode index page design

* Responsive auxiliary nav with dark mode toggle

* Remove box shadow when on smaller displays

* Prevent default on toggle click

* Move attribution/licensing to README.md

* Fix header padding on index page

* Background color transition animation

* Remove margin on index when on mobile displays

* Remove neon border when on mobile

* Fix post-header margin on mobile

* Lighten card headers in dark mode

svg/moon.svg

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<svg width="800px" height="800px" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path fill="currentColor" d="M12.0557 3.59974C12.2752 3.2813 12.2913 2.86484 12.0972 2.53033C11.9031 2.19582 11.5335 2.00324 11.1481 2.03579C6.02351 2.46868 2 6.76392 2 12C2 17.5228 6.47715 22 12 22C17.236 22 21.5313 17.9764 21.9642 12.8518C21.9967 12.4664 21.8041 12.0968 21.4696 11.9027C21.1351 11.7086 20.7187 11.7248 20.4002 11.9443C19.4341 12.6102 18.2641 13 17 13C13.6863 13 11 10.3137 11 6.99996C11 5.73589 11.3898 4.56587 12.0557 3.59974Z"/>
+</svg>