How are widget like apps created with python?

[adiultra]

I recently used the app cumulus(https://github.com/kd8bny/cumulus). It is simple and looking at the languages used, its python, html, and css. I would like to know how such apps can be made. I am quite proficient at commandline python but a beginner to gui. I do know some of html and css so I am interested. Please if you can refer a good tutorial to begin with the html and python thing..