Enhance and make dark mode support enabled by default

[gerteck]

Discussed in #2750

^{Originally posted by mendax1234 November 29, 2025}
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!