You could
- download materia-gtk-theme_20190201.orig.tar.gz from the Debian source tree
- unzip the package and run
bash install.sh -d /home/<USER>/.local/share/themes
(domkdir /home/<USER>/.local/share/themes
if the directory doesn’t exist) - install the Flatpak stuff for Flatpak applications
This should work similar with the Paper icons (/home/<USER>/.local/share/icons
).
This will install the theme/icon files for the current user, which should be okay, because no one should run GUI applications with root
.