Update code block colors

[tynandebold]

Description

Resolves #131

Development notes

Updated the colors to match Steph's guidance.

QA notes

Check the post here: https://deploy-preview-146--kedro-website.netlify.app/blog/a-polars-exploration-into-kedro

Checklist

• [ ] Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• [ ] Added tests to cover my changes, where applicable

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