Improve code syntax contrast

[eosfor]

Summary

• switch Hugo/Chroma highlighting from the light GitHub theme to github-dark
• keep highlighted code blocks consistent with the existing Prism Tomorrow token palette
• update the theme comment to reflect that highlighted blocks no longer always use a light background

Testing

• npm run build
• verified the affected PSRule article renders code with a dark background and high-contrast text

Closes #74

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[HeyItsGilbert]

Thanks for the fix!