fix: improve tutorial dark mode contrast

[Sean-Kenneth-Doherty]

Summary

Partially addresses #627 by cleaning up two dark-mode contrast issues in tutorial examples.

• make the unresolved $env/static/public preview use the tutorial foreground/background CSS variables instead of hard-coded white/black
• make the raw-state SVG labels inherit the current foreground color in both unresolved and resolved examples
• keep the tutorial instructions aligned with the new unresolved fallback values

I left the kitten image/background item out of this PR because that needs asset/background cleanup rather than a simple contrast fix.

Validation

• SHARP_IGNORE_GLOBAL_LIBVIPS=1 mise exec node@24.15.0 -- pnpm install --frozen-lockfile
• mise exec node@24.15.0 -- pnpm prettier --write <changed files>
• mise exec node@24.15.0 -- pnpm --filter svelte.dev exec node --input-type=module -e "compile changed Svelte files"
• git diff --check
• mise exec node@24.15.0 -- pnpm --filter svelte.dev lint
• mise exec node@24.15.0 -- pnpm --filter svelte.dev check
• browser smoke check against local dev server: dark raw-state SVG text fill matched rgb(230, 230, 230) and dark env-static-public main colors resolved to rgb(46, 46, 46) / rgb(230, 230, 230)

[vercel]

@Sean-Kenneth-Doherty is attempting to deploy a commit to the Svelte Team on Vercel.

A member of the Team first needs to authorize it.

[Sean-Kenneth-Doherty]

Local validation for 4ac2eeecee36a5d2e57cb063ddc055aa154a8d2e:

• SHARP_IGNORE_GLOBAL_LIBVIPS=1 mise exec node@24.15.0 -- pnpm install --frozen-lockfile passed.
• mise exec node@24.15.0 -- pnpm prettier --check <changed files> passed.
• mise exec node@24.15.0 -- pnpm --filter svelte.dev exec node --input-type=module -e "compile changed Svelte files" <changed Svelte files> passed for the three changed tutorial components/pages.
• mise exec node@24.15.0 -- pnpm --filter svelte.dev lint passed.
• mise exec node@24.15.0 -- pnpm --filter svelte.dev check passed with svelte-check found 0 errors and 0 warnings.
• git diff --check passed.

Browser smoke: I started the local dev server, set sv:theme=dark, and used headless Chrome 146/CDP to inspect the rendered tutorial previews.

• /tutorial/svelte/raw-state: the result iframe SVG text nodes (50, 60, $SVLT) computed fill: rgb(230, 230, 230).
• /tutorial/kit/env-static-public: the WebContainer preview loaded with ?theme=dark; the rendered <main> computed background-color: rgb(46, 46, 46) and color: rgb(230, 230, 230).

The Vercel status remains blocked on team authorization, so this is local validation only. The checkout is clean after validation.