Thanks for your answers.
The problem is that Hook::exec('Header'), or Hook::exec('displayHeader') don't do anything in my custom front controller. I can't include other modules' CSS or JavaScript files.
And when I try to var_dump(Hook), I just get the string 'Hook' and when I try to see if the function exec() exists php says it doesn't.
I have two files where I can use Hook::exec(). One extends ModuleFrontController, and the other extends ProductPresentingFrontControllerCore, why can't I use the Hook constant?
As an example, the shopping cart doesn't work because it can't load its own JavaScript files...
Thanks in advance,