Forms: forward-compatible WPDS color token fallbacks for @wordpress/theme 0.16

[CGastrell]

Proposed changes

The Forms dashboard SCSS references the WPDS color tokens by their pre-0.16 names (--wpds-color-bg-* / --wpds-color-fg-*). @wordpress/theme 0.16 renames these (bg- → background-, fg- → foreground-) and drops the old names entirely (WordPress/gutenberg#79098). So once the bundled @wordpress/* monorepo update (#49272) lands theme 0.16, these custom properties go undefined and the dashboard theming silently falls back to its hardcoded hex values (losing themed/dark surfaces).

This wraps each of the 15 references (6 dashboard SCSS files) to try the new name first, with the old name (and existing literal fallback) as the inner fallback:

var(--wpds-color-background-surface-neutral-strong, var(--wpds-color-bg-surface-neutral-strong, #fff))

• Valid on both the currently pinned @wordpress/theme 0.15.1 and 0.16 — on trunk the new name is undefined so it resolves to the inner fallback, i.e. the exact original expression. Computed values are byte-identical today; there is no visual change on trunk. It simply keeps theming working after the bump.
• --wpds-color-stroke-* tokens are unchanged in 0.16, so they are left as-is.
• Decouples the Forms slice from the bundled-monorepo bump — no dependency in either direction, and nothing here touches that PR.

Related product discussion/links

• WordPress/gutenberg token rename: #79098

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• On current trunk (@wordpress/theme 0.15.1): open the Forms admin — Responses/Inbox dashboard — and confirm it renders identically to before (surfaces, headers, tab colors, comment cards). The old token names still resolve through the fallback, so nothing should look different.
• Forward-compat check: with the bundled @wordpress/theme 0.16 bump applied (e.g. the Update Bundled @wordpress/* monorepo #49272 branch), confirm the dashboard still themes correctly — now via the new --wpds-color-background-* / --wpds-color-foreground-* names — instead of dropping to hardcoded hex.
• pnpm jetpack build forms compiles cleanly.

[github-actions]

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[github-actions]

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Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

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