feat: add dark mode

[w3cj]

Added dark mode.

• Detects user's preferred theme preference
• Loads hljs theme based on theme preference
• Updates stackblitz embed theme if user theme preference changes

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[saurabhdaware]

Thanks for your work on this feature @w3cj . Definitely a big help. I'll merge this into a separate branch and add more theming options on top of this :D