Can working privately in your own time for a business customer to save them money and to supplement your own personal income be considered an example of theft or fraud at work?

This might not be theft or fraud in legal terms, but it is an example of unprofessional and unethical behaviour. It is unethical because your employer is being deprived of legitimate business that they have built up as a consequence of your acting for your employer's client in this private capacity. It is also unethical because many of the skills you possess to carry out this work will have probably been gained from the training and experience provided by your employer. You may also be in breach of your contract to work privately, particularly in a situation where there is a conflict of commercial interest between you and your employer.