Update docs syntax colors head of dark mode

[mdo]

This PR updates the docs syntax CSS to what I use on <markdotto.com> ahead of shipping dark mode, which requires wider usage of CSS variables. Not a huge departure, and still something I'd like to tweak over time mind you. Open to feedback here still.

/cc @XhmikosR

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Cherry-picked from #35857 at abba53d (which is why I named this branch dancing-queen lol).

[XhmikosR]

Are you sure you are not missing any classes compared to before? This is not the same as Jekyll which is one of the reasons I had kept the comments. I'm fine with dropping the comments assuming we still include all classes.

Also, note that the new colors are not WCAG2AA valid and I had spent some time to make them valid before :/

PS. we use the same file in blog and icons too so it probably makes sense to fix any issues before landing this and backporting it.

[patrickhlauke]

the problematic colour combinations seem to be centered around the code highlighting styles

[mdo]

Good callout, I didn't realize those were so bad. I've darkened a couple colors to fix the most obvious issues. Going to double check the rest shortly.

[patrickhlauke]

confirming that i don't note any immediately apparent contrast issues on https://deploy-preview-35984--twbs-bootstrap.netlify.app/

[mdo]

Going to merge as-is for now and we can iterate on colors if needed in future updates.