Add a background color to good/bad codeblock examples

[queengooborg]

This PR adds a green and red tint respectively to the background of good/bad example codeblocks. This helps distinguish good vs. bad examples much better, as the icons in the corner are easily missable at a glance.

Before:

After:

I tested this in both dark and light theme, and under various colorblindness types -- this change does not negatively impact colorblind readers, and only either improves the experience or has no impact.

[caugner]

@queengooborg Could you please add screenshots for the light theme as well? I will then ping our product designer to review the changes.

[queengooborg]

Sure thing, got a screenshot ~~after searing my eyes from the burning light~~ in light mode here:

[caugner]

Fixed https://github.com/mdn/yari/issues/5686.