Nav link contrast with background/other text too low

[ikirker]

While looking at the code snippet colours, @dstansby posted this link to an accessibility analysis tool which suggested that the contrast on the navigation links may be too low.

Worth taking a look in both dark and light mode to see if we should override these, probably using the SCSS variables, e.g. in https://github.com/UCL-RITS/mkdocs-rc-docs/blob/master/mkdocs-project-dir/docs/stylesheets/tweaks.css.