From scratch? yes extremely hard and it would take a team of highly skilled individuals to create one. However i'd suggest getting yourself familiar with the *nix environment and start playing with configs there. Teach yourself to know how to change every aspect of it and eventually you could start with a fresh install maybe pop gnome on it and create a customized "OS". Keep in mind the theme and look isn't the only thing that makes an OS though, you'll want to add your own utilities and processes which makes yours unique.