Repairing blank screen and replacing Gnome with Mate
In your home directory, move old configs:
mv ~/.config ~/.config.old
mv ~/.local ~/.local.old
mv ~/.cache ~/.cache.old
Uninstall Ubuntu desktop:
# apt-get autoremove ubuntu-desktop-minimal gnome-shell
Uninstall GNOME Display Manager:
# apt-get autoremove gdm3
Install Mate Core Desktop:
# apt install ubuntu-mate-core
Reboot