The package matcha-gtk-theme-20200322-1-any.pkg.tar.zst has broken some things in the Matcha-aliz theme, particularly within Firefox 74.0 (firefox-74.0-2-x86_64.pkg.tar.zst). Specifically, something in the file /usr/share/themes/Matcha-aliz/gtk-3.0/gtk.css has changed so that selected text in Firefox is no longer shown inverted, thus rendering selections "invisible" to the user. Both the Matcha-azul and Matcha-sea themes seem to work fine.
Package matcha-gtk-theme-20200321-1-any.pkg.tar.zst has the same problem. Reverting back to package matcha-gtk-theme-20200315-1-any.pkg.tar.zst fixes the problem.
I am examining diffs of the two gtk.css files, but honestly my ignorance of CSS makes it difficult to see what's wrong. Perhaps one of you CSS wizards can spot the problem immediately.