Mice can be fed a commercial complete ration supplemented with occasional very small treats of vegetable.
They are happy to eat tit bits and chocolate but these are not suitable snacks and they can quickly lead to obesity and considerable health problems.
Avoid using hamster foods as these tend to be too rich for mice, which is often linked to skin problems. Instead use a feed that is designed specially for mice.
Commercially produced treats are generally best avoided, healthier snacks being far preferable.