The Notepad theme for Grav is a direct port of the Notepad Theme for Jekyll designed by Hossain Mohd Faysal.
You can download the theme here: github.com/getgrav/grav-theme-notepad but it's strongly advised to download the complete Download Notepad Skeleton.
You can find out more info about customizing your Grav theme, as well as basic Grav usage documentation at learn.getgrav.org
You can find the source code for Grav at github.com/getgrav/grav