DOC: WEB: dark mode theme in docs when user has browser in dark mode

[anudeepsamaiya]

Location of the documentation

Overall the docs should be able to support browser theme selected by user. Or the user should be able to switch the theme as per preference.

Note: You can check the latest versions of the docs on master here.

Documentation problem

It would be nice to support the dark mode with toggle button on top header.

Suggested fix for documentation

https://github.com/MrDogeBro/sphinx_rtd_dark_mode (there are several sphinx plugin to do, this is just an example.)