chore(web): remove stale theme artifacts

[floze-the-genius]

Closes #826.

Summary

• remove the unused PaletteIcon export
• update the appearance preview comments for the current four-theme set
• replace the obsolete Tailwind config reference with the actual dark custom variant
• document the terminal theme's current registration/import/font wiring

Validation

• npm run check:nul
• npm run typecheck (Node 22)
• npm run test:web — 1,791 passed
• npm run build (Node 22)
• npm run test:e2e — 90 passed, 2 skipped, with one unrelated sub-agent timing failure; the exact failed scenario passed 1/1 when rerun in isolation
• git diff --check

[edspencer]

Thanks @floze-the-genius!

I do believe you are the first contributor 🥳