Add theming, and color-deficiency-friendly themes

[kylesferrazza]

• customizable theme site-wide, dropdown in header or similar (and check prefers-color-scheme media query)
• store user theme prefs in localStorage
• dark mode
• color-deficiency-friendly themes that change green/red usage in rubrics and comments

[blerner]

https://github.com/brownplt/code.pyret.org/pull/390 has a bunch of useful colors we can use...