[Docs]: Add support for dark theme.

[Evanroby]

Description of the changes

Adds dark mode support for the docs using prefers-color-scheme: dark. Styles most things such as layout, sidebar, code blocks (Monokai), tables, admonitions, etc.

Have the changes in this PR been tested?

Yes

[EternalllZM]

Looks great, what a pleasure to read. There are a few things however I could see being an accessibility/visibility issue.

1. The commented out lines of code are a little too dark.

2. The "Important"/"Tip" highlights are a little too bright and thus contrasts badly against embedded code.

3. "Warning" does not draw enough attention to potentially dangerous actions taking place.

My opinions

Commented out code can use a soft blue

Important/Tip highlight could look like this

[Evanroby]

All requested changes have been implemented !