feat: dark mode

[jsilvela]

Do dark mode, right this time. Last time around there were some problems and it had
to be rolled back:

• dark background was a not-very-dark blue, forced because the CNPG logo is dark. This was uncomfortable for some people
• no opt-out switch

We have CNPG logos in white in the https://github.com/cloudnative-pg/artwork repo.
This should give us more latitude to use dark backgrounds.

This is work in progress. References

• Switching: https://www.freecodecamp.org/news/how-to-build-a-dark-mode-switcher-with-tailwind-css-and-flowbite/
• Tailwind color reference: https://tailwindcss.com/docs/background-color
• Tailwind dark mode reference: https://tailwindcss.com/docs/dark-mode
• Palettes for inspiration: https://colorhunt.co/palettes/dark