Is this closed source?

[gorenbk]

[probonopd]

This is open source. You can learn more about it here: https://hellosystem.github.io/docs/

And you can find the source code here: https://github.com/helloSystem/

[gorenbk]

All I can find is the ISO file or the branding things.

[probonopd]

All code is there. Can I help you locate the code you are looking for? What exactly are you interested in?

[gorenbk]

Where is the code for the GUI?

[probonopd]

Which component do you mean by "the GUI"? The file manager? The menu? The utilities? They all have their own repository.

[gorenbk]

The GUI behind all that: for how any application is displayed.

[gorenbk]

Not for the background, menu, etc. Just for the border around the application that says it's name.

[probonopd]

This is the source code for the window decorations (like the title bar of each window): https://github.com/helloSystem/BreezeEnhanced