So if you have two fields - first name and last name, then you should do in the css object:

css={{
  form: {
    display: "flex",
    flexDirection: "column",
    "& > div:nth-of-type(3)": { // first name
      order: -2
    },
    "& > div:nth-of-type(4)": { // last name
      order: -1
    }
  }
}}>