Added dark mode functionality

[amit-077]

Added a toggle button on left top corner, so user can switch between dark and light modes.

[lgarron]

I appreciate the instinct to contribute, but I'm concerned that:

• This reformats way more than necessary, making it unclear how much actually changed (and affecting future diffs).
• The implementation isn't really appropriate for 2024. If we're going to do dark more, I think it should use prefers-color-scheme and color-scheme features of CSS.
• We have more than one page now.

[amit-077]

Are you seeking to implement a theme that automatically adjusts to the user's system preferences for light or dark mode, without the need for a manual toggle button?

[lgarron]

Are you seeking to implement a theme that automatically adjusts to the user's system preferences for light or dark mode, without the need for a manual toggle button?

I'm not particularly convinced dark mode is the right choice for this site, as that would vary the visual cues it presents in testing and screenshots.

But yeah, if it's going to have dark mode I think it should use modern conventions and match the OS. If there's an override, it should be an optional override config in e.g. localStorage that is used to set an attribute on the body to set the color scheme using CSS cascading.

[amit-077]

I have added dark mode support with OS preference detection and optional localStorage override. Please review it.