[Dark mode] Left-nav color and contrast adjustments are needed

[chalin]

• Parent issue: Support dark mode #331

• The left-nav title ("Documentation" in the image below) contrast isn't great enough (it fails https://webaim.org/resources/contrastchecker). Screenshot from the UG:

  

• The icon for collapsed left-nav menu entries aren't visible unless hovered over. Besides, the contrast isn't good either way. Screenshots taken from https://deploy-preview-4347--opentelemetry.netlify.app/docs/:

  Light	Dark*

  ^* Hover is active over Getting started