DOC: WEB: dark mode theme in docs when user has browser in dark mode
#### 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](https://pandas.pydata.org/docs/dev/).

#### 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.) 
