Description of your changes

We had a custom css for code highlight in light mode which was also applied in the dark mode, making the code unreadable.
This issue has been fixed recently in this PR. We can now upgrade MkDocs to 9.3.2 and apply custom CSS only to light mode, leaving dark mode code highlight as default.

Related issues, RFCs

Issue number: closes #1693

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my change is effective and works
• The PR title follows the conventional commit semantics

Breaking change checklist

Is it a breaking change?: NO

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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