fix: prevent code blocks flash in dark mode (AI-assisted)

[titojuanc]

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• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Fixes #11566

When loading a Docusaurus page in dark mode, code blocks briefly flash with light theme colors before switching to the correct dark theme. This happens because Prism theme colors were applied as inline styles via JavaScript, which only take effect after React hydration.

This PR injects CSS variables for both light and dark Prism themes directly in a <style> tag in the , leveraging the [data-theme] attribute that is already set by a blocking script. This ensures code blocks display with the correct background color from the very first paint, without waiting for hydration.

Test Plan

1. Configure a site with different light/dark Prism themes (e.g., oneLight / oneDark)
2. Switch to dark mode
3. Hard-reload any page with code blocks
4. Observe that code blocks no longer flash with light theme colors
   (AI-assisted)

Test links

Related issues/PRs

Closes #11566

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