I think you’re remembering our discussion over here. As @DaveSch has noted, you want the Themes (plural) add-on, which will then, in typical fashion, add a tab to the preferences called “Theme” (singular) and show up in the Plugin Manager (which I also recommend installing) as “Theme preferences”. 
On my 2017 MBP I use the Adwaita theme with 14-pt Helvetica and “Toolbar text” checked, and find it readable.