revised toggleable light mode/dark mode

[shiftingtech]

Based on the work of GadgetAngel, but revised for mantainability.
Also draws on
https://derekkedziora.com/blog/dark-mode-revisited

This may still need some tweaking. Primarily, I'm not happy with the current location of the dark/light toggle

[nemgrea]

do you want to incorporate anything from this most recent issue into this?
#316

[shiftingtech]

I'm not sure any addition is necessary. (though the suggestion of making the color names a bit clearer is a good one). I think the original concept from GadgetAngel's version was to have the light theme be the color blind friendly one, which makes as much sense to me as anything...