For the record, I have never tried "GTK+3 Classic" - https://launchpad.net/~lah7/+archive/ubuntu/gtk3-classic - created by Luke Horwell (@lah7 here in the "Ubuntu MATE Community"), but I think that I can help here a little bit, even so.
I see that in the web page "GitHub - lah7/gtk3-classic: Patches to bring back a traditional experience for GTK+3" there are more specific instructions to install "GTK+3 Classic", specifically the following instructions:
" (...)
Installation
(...)
Ubuntu 20.04, 21.10, 22.04
These packages are based on the original
gtk+3.0
package, with the version suffixed with~classic
.To install, add the repository and upgrade your packages:
sudo add-apt-repository ppa:lah7/gtk3-classic sudo apt dist-upgrade
Available for these architectures: amd64, arm64, armhf, i386, powerpc and ppc64el.
To restore the original GTK3, use
ppa-purge
to restore the packages from Ubuntu's repository:sudo apt-get install ppa-purge sudo ppa-purge ppa:lah7/gtk3-classic
(...) "
I hope that helps You may also want to read the following 2 related posts written by @lah7 here in the Ubuntu MATE Community: