If you want to build your custom version of Moodle Desktop, please follow the Electron Guides that you will find in https://electron.atom.io/docs/
Please, consider the following:
- For building a Windows version you will need a Windows computer.
- The Mac version will require a Mac computer.
- The Linux version may require a Linux distribution (depending on the selected target).
Note also that if you want to publish your custom versions in the Windows or Apple stores, you will need to sign-up for a developer account (it costs money).
Moodle HQ provides a branding service of Moodle Desktop via the The Branded Moodle Mobile App. This service includes publishing in the stores and code signing.