First of all, install all of the themes to your darktile config directory using:
cd ~/.config/darktile
git clone git@github.com:liamg/darktile-themes.git
Then select the theme you want to use and create a symlink:
# backup an existing theme if you have one
mv ~/.config/darktile/theme.yaml ~/.config/darktile/theme.yaml.backup
# set the desired theme
ln -sf ~/.config/darktile/darktile-themes/themes/3024_Night.yaml ~/.config/darktile/theme.yaml
To change themes in future, just overwrite the symlink, like this:
ln -sf ~/.config/darktile/darktile-themes/themes/3024_Night.yaml ~/.config/darktile/theme.yaml