Lightly adjust font colors for code blocks

Resolves #990 (closed)

I think this is a better MVC for the contrast issue with code blocks. I used the Wave accessibility tool to more carefully figure out which specific colors were the issue, and then I used https://webaim.org/resources/contrastchecker/ to tweak the colors slightly, to improve the contrast. It's a small tweak, but I definitely prefer it. It fixes the warnings, and won't be as big a change as the other POCs I tried.

The red is lightened slightly, and the dark code comments are lightened slightly as well. This now passes the Wave tool testing.

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• After: