Document theme colors in custom utilities

[PedroLunet]

The functional utility documentation explains theme matching with tab sizes, but does not show how to use theme colors. Add a glow-* example using --value(--color-*), with HTML showing a color from the default palette and a custom color defined with @theme.

Addresses the missing color example reported in #2473.

Test plan

• corepack pnpm exec prettier --check src/docs/adding-custom-styles.mdx
• Compiled the exact CSS snippet and HTML classes with @tailwindcss/node using Tailwind CSS 4.3.3. Verified that glow-blue-500 and glow-avocado-500 generate box shadows using the expected theme variables, and that both color values are emitted.
• Ran corepack pnpm dev --hostname 127.0.0.1 --port 3100 and checked the rendered documentation in the browser, including CSS and HTML highlighting. No browser console errors.

[vercel]

@PedroLunet is attempting to deploy a commit to the Tailwind Labs Team on Vercel.

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[greptile-apps]

Confidence Score: 5/5

The documentation change appears safe to merge.

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