I just had the same question and looked into it. The answer is no you can't, they removed the ability to disable Proton as of version 91.

I also strongly dislike the new tabs as they're totally disconnected from the actual pages they're a tab for with whitespace, less contrast as you mention as well which an issue if you don't have great eyesight. I guess they're willing to lose even more market share over this, which seems pretty silly considering what their trajectory has been...