Add dark mode theme

[alvarocastro]

Would be great to have a theme for dark mode to be in line with other docs sites.

Also this might include a switcher in the header or floating button in the corner, or just sync with the client's browser/os setting.

[RobbieTheWagner]

@alvarocastro totally agree with you! I have not had the time to work on updating Tailwind, but if we could get on latest Tailwind, it would be fairly trivial to add dark: styles to all the things. Would you have any interest in helping out with this effort?