what can I feed?
As mice are not very picky there is a lot that you can give to your mice, but I don't recommend giving them a lot of "people food" as this food has much of unnatural addings, like extra salt, fat, other spices and sweeteners. This is not very good for us humans but especially not for your little pets as they eat a lot more of it in proportion then us.
I always give my mice chunks, they have everything they need in them, I use the brand Versele lage rat & mouse pro, they like this food very much. With that I give them a rodent mix from Garvo every two days and some ( dried) herbs, seeds and (dried) fruits occasionally. The herbs have some extra value as they help the health a little bit at some points, like, garlic is good against parasites and several herbs are anti bacterial, some help clean the blood, or help their furr condition etc etc.
Don't I ever give something from our kitchen? Yes, I do! But you have to be cautious about what to give. I sometimes give them dried bread, make sure it is dried good as the enzymes can be harmfull for the mice. You can give some cooked potatoe, cooked rice, cooked pasta, but make sure you didn't add anything to it. Also a little piece of for example an apple, banana or pear is ok and some vegetables or ok too. But I won't advice non dried fruits and vegetables as they could get diarrhea when they have a bit too much.
Other treats to give are for example meal worms, they love it! And it it good for them to have some extra proteins. They also like millet, sunflowerseeds and peanuts, but make sure you don't give too much of anything as also mice can get fat!