Update code highlighting color scheme

[Siddhant-K-code]

Updates the code highlighting color scheme to a new set of colors based on community voting (https://github.com/ocaml/ocaml.org/issues/1772#issuecomment-1813439004) and WCAG contrast requirements.

• Changes color values for keyword, constant, function-binding, variable, support, comment, literal, string, and numeric classes to new, WCAG-compliant colors.
• Maintains the existing diff-related styles without changes.
• Refer to stephango.com/flexoki for color styles.

fixes https://github.com/ocaml/ocaml.org/issues/1772

[cuihtlauac]

@Siddhant-K-code Don't worry about the failed test, that's not on you. It doesn't matter. It's there because it was in main when you created your branch. You can rebase to make it go, but it's not required.

[Siddhant-K-code]

I've rebased it with the latest changes in main, hope it should be fine now to merge 🤞🏼