docs: increase syntax highlighting color contrast

[geoffrich]

This partially resolves one of the Axe Accessibility violations listed in #5678: Elements must have sufficient color contrast (2018 count).

This reduces the number of color contrast violations from 2018 to 228. The colors for the syntax highlighting tokens were below the WCAG AA threshold of 4.5. This meant that people with low vision could have difficulty reading the text. Below is a list of contrast ratios for each token, before and after the change.

• base: was 4.26, now 7.21
• comment: was 2.8, now 5.23
• keyword: was 4.1, now 4.54
• function: was 4.31, now 4.5
• string: was 3.49, now 4.65
• number: was 2.84, now 4.89

Before:

After:

I adapted some of the colors from Eric Bailey's accessible syntax highlighting themes.

[benmccann]

It would help get this reviewed to include a screenshot of the changes

[geoffrich]

It would help get this reviewed to include a screenshot of the changes

I put a before/after image of one of the syntax highlighted code blocks that shows the changes -- was there something else you wanted a screenshot of?

In the image, you can see that (for instance) the text color of the comments in the "after" image is darker than the text color in the "before" image.

[benmccann]

Oh, I didn't look that closely and didn't realize that you were changing the colors for code blocks. Sorry for the confusion 😄