Add support for dark mode

[nirban256]

Page https://redis.io/docs/latest/

It will be better if the support for dark mode is added as it improves the readability, design and provides flexibility to the users as they get the option of choosing mode they want to use the website in, especially for the docs part.

It is a suggestion by me, I would like to work on the issue, if possible and I am open to feedback or any other suggestion.