Add dark theme for docs

[thrau]

Users have requested dark themes for the webapp and the docs:

• feature request: add dark mode localstack#7853

[Oxymoron290]

My retinas are burning!

[Oxymoron290]

google/docsy#1909

[Oxymoron290]

https://www.docsy.dev/blog/2024/0.10.0/#color-themes-and-dark-mode-support