Add theme cards, listings, and specs

[burieberry]

Adds a set of new themes to the catalog:

• 12 new Theme/ card instances: amethyst-haze, botanical, catppuccin, claymorphism, mono, nature, neo-brutalism, sage-garden, soft-pop, starry-night, supabase, winter-wonderland; substantially rebuilds catalog-storefront, updates boxel-brand-guide and cardstack-brand-guide, and removes the superseded Theme/cardstack.json
• 15 new ThemeListing/ instances so the themes show up in the catalog app
• 4 new Spec/ cards: theme, structured-theme, brand-guide, style-reference

Also fixes spec-fields-grid.gts styling to use theme CSS variables (--border, --card, --card-foreground, --muted-foreground) without hard-coded fallbacks, and lets field rows wrap.

Linear: CS-12593

🤖 Generated with Claude Code

[github-actions]

Staging Submissions Preview

This PR's content is pushed to the staging submissions realm: https://realms-staging.stack.cards/submissions/

Changed folders:

• Spec/
• Theme/
• ThemeListing/
• boxel-surface/
• catalog-app/

Updated at 2026-08-26 23:15:12 UTC for commit 3c3cb41. Shared realm: only this PR's changed files are pushed; files touched by multiple PRs reflect whichever pushed last, and deleted files are not removed.