General Styling Improvements

[thousand]

Seeing #59 and noticing a lack of dark mode support, I thought I'd offer to do a pass over the styles to make these and a few other improvements in a batch.

• Address #59 and make some other legibility tweaks
• Refactor and modernize the CSS to reduce the page weight (not that it's huge) and make future changes easier
• Add a dark color scheme and support for system dark mode (and/or provide a manual toggle)
• Make the main content and book page more visually consistent

Let me know how this all sounds, if there's any guidance or patterns i should follow, and if there's anything else that could be done.

[lethain]

Hi @thousand -- I'd be quite grateful if you submitted a PR that handled those. I don't have any strong points of view other than trying to keep submission process simple (e.g. just adding a markdown file) which I don't imagine your proposed changes would impact.