I think Chrome OS is based on Linux and written in C/C++. I haven't looked at the source but presumably it wouldn't be too hard to change some of the images and UI etc., but if I wanted to, for example, make it log into a proprietary account instead of Google's accounts on login, how hard would it be?
How about pulling down the source first and trying to build/install it yourself? If you feel that it's way overwhelming, then nobody can say it's easy for you.
It's hard to answer "how hard" because there's not an exact standard about "how intermediate" or "how fast can you learn".