You'll need the dconf command to install the theme. If you're using elementary OS (or another Debian-based distro), you can install it by running:
sudo apt install dconf-cli
After cloning this repo, or downloading and extracting a ZIP archive of the code from GitHub, change into the project directory and run the install script.
cd pantheon-terminal
./install.sh
The changes should take affect immediately in the terminal window.
The install script supports a quiet option that suppresses all info messages and prompts.
./install.sh --quiet
# OR
./install.sh -q
To uninstall the theme, just use the --uninstall flag. You can also use --quiet to uninstall without info messages or prompts.
./install.sh --uninstall
# Shhhh...
./install.sh --uninstall --quiet
The changes should take affect immediately in the terminal window.