add dark mode support

[joshka]

• added dark mode styles
• minor fix
• feat: added toggle theme button
• fix: toggle button and navbar style fixes
• fix: color contrast fixes
• footer color contrast issue
• fix: :bug: empty white space on large resolutions in dark theme

Cherry picks commits from #701 to fix the merge

[joshka]

Bulma 1.0 has dark mode support built-in, but is a little complex to upgrade. See:

• https://github.com/tokio-rs/website/pull/793