To install this theme you will first need to make sure ~/.warp/themes is in your home (~) directory. If it is not you can add it by running the following:
mkdir -p ~/.warp/themes
Next you can download this theme into the themes directory by running the following command from your terminal.
curl -o ~/.warp/themes/hisoka.yaml https://raw.githubusercontent.com/zpthree/warp-hisoka-theme/master/hisoka.yaml