Add dark mode styling

[extremelyonline]

Hello, this PR add dark mode styling:

• Put all colors into variables, defined in base.css
• Pure css, use prefers-color-scheme to set light/dark mode
• Light and dark mode share the same primary color #6b7280, svg colors are unchanged
• Dark mode colors are in the same shade as #6b7280
• Remove all box shadows in dark mode
• There is no toggle button (yet)
• Perhaps an environment variable to force light/dark mode can be added?

Closes #22

[syeopite]

Thanks this is awesome! I'll take a closer look at this soon but do you mind if this is pushed to the next release?

I'm planning on having a proper "0.1.0" release in the coming days so I'll only be doing bug fixes for now.

I'd like for this to be a "0.2.0" feature which should hopefully come with a settings page that'd go nicely along with this feature.

[syeopite]

Hello,

Sorry about the inconvenience but I'm superseding this PR with #35. There's been a substantial amount of changes in the CSS since last December, and along with the changes in #35 makes this PR extremely difficult to merge.

However, I've marked you as a co-contributor in the relevant commits relating to dark mode and CSS variables in #35.

Apologies for not being able to mark this as merged :(