Aside from the inexperience issue (D7 and later really do give you the power to override theming on just about anything), you should never promise a client that

Aside from the inexperience issue (D7 and later really do give you the power to override theming on just about anything), you should never promise a client that a web design will replicate a mock-up with pixel perfection. It's impossible; there's just too much that's out of your control. A web design mock-up is more of a guideline that can be followed pretty closely, but never perfectly replicated.