How to contribute to a project where the repository only contains .patch files?

I’m trying to contribute to Zen Desktop (a browser based desktop environment, dev branch), but the repository only contains .patch files across all branches : no original .js/.css source files. For example:

src/browser/components/panelUI.css.patch  
src/browser/themes/shared/tabs.css.patch  

I do see .css and .js files but with .patch at the end and this is same everywhere.

I read the contribution and installation guidelines but I do not any explicit documentation on patch management.

How should I set up a development environment if the repo only provides patches? Are there any commands that I need to execute before I can start contributing?

Link: https://github.com/zen-browser/desktop

Grateful if any of you can help me!