A11Y: change link text color for dark mode

The link text color in dark mode doesn't provide enough contrast with the background (checked with webAIM's a11y tool). Given a lot of people use dark themes, mdBook should change the css for dark mode themes.

Relevant discussion: https://github.com/rust-lang/mdBook/issues/1646 and https://github.com/rust-lang/mdBook/issues/1915