Comments in code blocks are almost invisible

[36degrees]

Description of the issue

Comments in code blocks are now #eee on #f9f9f9, with a color contrast of 1.1:1, and are pretty much invisible

I'm pretty sure this is a recent regression, possibly introduced as part of an update to the tech docs gem.

Steps to reproduce the issue

1. Go to https://deploy-preview-615--govuk-frontend-docs-preview.netlify.app/import-css/#import-specific-parts-using-sass
2. Look at the code block
3. Look even closer – I promise there are comments there

Actual vs expected behaviour

You should be able to read the comments.

[NickColley]

The immediate issue is resolved by #621 but will leave this open until the fix is upstreamed into the tech-docs-gem