Add dark mode support

[mendax1234]

Hi Developer team,

Thanks for making this awesome website, which is used by some SoC courses at NUS. However, will it be possible to add support for the dark mode? As for now, many document websites like mkdocs, gitbook, etc, support dark mode alr. Hope markbind will support it also!

Thanks!

[damithc]

Good suggestion, @mendax1234