feat: dark mode theme

[delta1]

Looking at the RPC docs on a light background was wrecking my eyes :sweat_smile:

This PR:

• adds dark color theme for users with prefers-color-scheme: dark
• increases max width to 1200px
• replaces logo png with svg - black on light, white on dark theme. Smaller filesize too.
• removes !important qualifiers that make editing styles more difficult
• stores colors in scss variables for ease of maintenance

Deployed example at https://elements.delta1.dev