I encounter the issue as a visitor 
And it doesn’t affect all the themes, as you described in your first post.
Edit: I changed a bit my theme settings in https://meta.discourse.org/my/preferences/interface, and I’m now encountering the same issue as @merefield logged in as well.
Could it be related to the “Dark mode” setting in our preference? If set to “Dark”, then the regular Grey Amber won’t be applied 
?