Hello, friends. Just a quick follow-up: Using TCL 10.x's version of gtk3 in TCL 11 fixes the hard-to-see checkmarks, but some icons (e.g., those in the path/filename selection box that pops up when downloading a file in firefox) still appear distorted.
To fix the distorted icons, I also had to downgrade adwaita-icon-theme.tcz to the version in 10.x.
So to summarize, for gtk3 to look nice in Pure64 11.0, I'm having to use gtk3.tcz and adwaita-icon-theme.tcz from 10.x.