Base16 for Gnome Terminal is a collection of gnome terminal color profiles.
Firstly, clone the repository:
git clone https://github.com/robotdance/base16-gnome-terminal.git
./install-one.sh ./profiles/[profile-name]
./install-all.sh
Next, restart or open Gnome Terminal (not always necessary). Right click on the terminal and select profiles the menu that pops-up. The scheme you just sourced should be available for selection.
Fork, do your changes, submit pull request.
For new themes you only need to submit a new color profile, look at ./profiles
and get a sample.