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