Styling: fix white link on background styling issue

[Blargian]

Summary

Closes #4604

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/integrations/index.mdx

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Pull Request Overview

This PR fixes a styling issue where links were appearing white on a white background in warning admonitions within the dark theme. The fix adjusts the CSS selector scope to ensure that links within dark-themed warning admonitions are properly styled with black text.

• Restructured CSS selector to nest link styling within the dark theme warning admonition block

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