Apple don't make it easy to make superficial cosmetic changes. The idea is so that anyone can sit down at any Mac and instantly find it familiar. (I have memories of working on offices on Windows computers that were so customised with pink and green, and stuff moved about, they were impossible to use.)
There are hacks for the system to tinker with things like this, but they are invariably unstable and need re-applying every system update. Modifying system components can (and increasingly will) cause trouble with the security signing.
IMO, it's not worth the effort just for the brief nostalgia that a rainbow apple will give you.