This is a template for base16 which supports xfce4-terminal
Put the prebuilt *.theme files from colorschemes/ into ~/.local/share/xfce4/terminal/colorschemes/ (create this directory if it does not exist), and the themes will be available under Edit > Preferences... > Colors > Presets
If you want to build the themes under colorschemes/ yourself,
a builder like this is required,
or use build.go.