Differentiate code highlighting based on light/dark browser theme

Currently, there is only one pygment code block themeing option, and there don't seem to be any pygment themes that are readable on both light- and dark-mode preferenced browsers. One will always be nigh-unreadable.

To fix this, I should provide a way to selectively load one based on the browser-requested theme, with a fallback for browsers that predate these preferences.