feat: create a functional dark mode

[a2937]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

Closes #828

Okay it seems to be adapting properly to the freeCodeCamp theme.

[a2937]

Oh the lock file. Oops

[raisedadead]

Looks great. I think we need to figure out a few kinks which can probably be handled while after PR lands?

Hey @scissorsneedfoodtoo 👋 !

How to handle this

Default image is has sam bd the publication bg

[raisedadead]

I updated this PR with several fixes and updates to polish it up. Mainly addressed:

• Adds dark/light theme toggle with system preference detection and localStorage persistence
• No flash of unstyled content (FOUC) — theme applied before first paint
• Proper Prism.js syntax highlighting with separate light/dark themes from CDN
• Toggle is an accessible with aria-pressed
• Background hierarchy: page gets --secondary-background, elevated elements (cards, code blocks) get --primary-background
• Bunch of new tests.

Some screenshots:

[a2937]

Thank you very much for your assistance.