Change theme is easy, first you must install a new theme in the template directory and after in administration, you can change the theme in you Design configuration

I suppose we must use the example template to create a new template ?

Hello Patrick

Ues you are right, you have 2 solutions,

Copy the default directory to inside another or take the files. After you can change everything.

Becarefull to the css, it must be inside the new directory.

Also if you create a module (and activate), it must be included inside the default directory because if you create another template without the files inside the default template you can have an error.

All the default files are overridden when you use another template.