Tweak blue color on dark page

[jayhesselberth]

Fixes #2732

[jayhesselberth]

I don't think tweaking each color is a very sustainable approach, but we could certainly recommend better colors.

[github-actions[bot]]

🚀 Deployed on https://66c3a0c631b8ce260195f952--pkgdown-dev.netlify.app

[hadley]

I think we probably could dig up the colours that RStudio or positron use for here. (Assuming we get them as names, which might be an incorrect assumption)

[olivroy]

Yes! Idon't know how to extract theme from cli, but it uses the Solarized Dark theme by default (https://ethanschoonover.com/solarized/)