Support for dark mode on documentation pages
Created by: donhuvy
Please has dark mode for document page at here
https://getbootstrap.com/docs/
https://getbootstrap.com/docs/4.3/getting-started/introduction/
(Dark mode at this site https://docs.microsoft.com/en-us/aspnet/core/getting-started/?view=aspnetcore-3.0&tabs=windows is a good example.)