style: match UniSC Athletics presentation

[Cozmocat]

The imported UniSC Athletics pages currently use the generic EmDash blog shell, which makes the migrated site look substantially different from the original WordPress site.

This PR rethemes the blog shell through templates/blog/src/styles/theme.css to match the original site's blue athletics header, typography, spacing, light content background, footer colors, and responsive mobile layout. Imported page content and navigation destinations remain unchanged, and the signed-in Admin link spacing is preserved.

What does this PR do?

• Adds UniSC Athletics visual theme overrides in the blog template stylesheet.
• Recreates the original blue header treatment and athletics-focused typography.
• Improves page width, content spacing, footer contrast, and mobile behavior.
• Leaves imported page content untouched.

Type of change

• Bug fix
• Feature
• Refactor (no behavior change)
• Translation
• Documentation
• Performance improvement
• Tests
• Chore (dependencies, CI, tooling)

Checklist

• pnpm typecheck passes — not run; CSS-only change
• pnpm lint passes — not run; CSS-only change
• pnpm test passes — not run; CSS-only change
• pnpm format has been run — not run; CSS-only change
• No translation changes are required
• No changeset is required for this CSS-only template fix

AI-generated code disclosure

• This PR includes AI-generated code — Codex / GPT-5.6 Luna
• I have read CONTRIBUTING.md

Screenshots / test output

CSS-only presentation change. The Cloudflare preview should be checked against the original site after deployment.