Redesign dark mode

[querkmachine]

Resolves #46.

Changes

• Changes dark mode colours to use deep purple and saturated green, an inversion on the light theme's colours.
• Changes colours of favicons to match the new dark mode scheme.
• Changes the footer collage background colour to the non-contrasting accent colour.
• Updates theme-colour meta tags to be dynamic based on prefers-color-scheme and to match the background colours of the light and dark themes.
• Updates webmanifest to have new theme_color and background_color, and to make these colours different.
• Updates the brand page to
  • show the new colours.
  • present colours according to their usage.
  • remove the contrast swatches.
• Tweaks tables so that headings are left-aligned by default.