To compile Pixelorama's source code, all you have to do is download Godot 4.5. You don't need the mono version since Pixelorama is written entirely with GDScript. Then, you clone Pixelorama from its GitHub repository, load the project within Godot, and run it! Simple as that!

If you wish to export Pixelorama as a standalone executable to a platform, you can refer to the relevant page of the documentation of Godot: https://docs.godotengine.org/en/stable/tutorials/export/exporting_projects.html