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.)