The code for the theme and plugin files is maintained in private repositories on GitHub. To work with the code you will need to have a reasonable understanding of working using git to commit and push changes, pull changes, create branches and merge changes.
The inbuilt git handling in VSCode makes this as easy as it can be. If you need a refresher there is a good series of videos on YouTube.
Some of the team use the command line – for this you will need to install the git command line client from here https://git-scm.com/downloads .
If you prefer, from the same location, you can pick any of the git gui clients.
Refer to the development process page for how we use github.