Install

[Bearded12]

I would extract Midnight-GrayNight into /usr/share/themes as root and the system cannot see it GTK 3.22 aswell, Ubuntu Mate 18.04.3

[chvz-creates]

The permissions might not be right. You can try the following commands and see if they work for you.

cd ~/downloads
git clone https://github.com/i-mint/midnight.git
cd /usr/share/themes
sudo cp -ar ~/downloads/midnight/* .

cp -ar makes sure that the permissions of the copied files are maintained.

Open tweaks and see if this worked.

then you can delete the repo

rm -rf ~/downloads/midnight