Customize padding Material UI Table - last-child

Thats the right approach, you just have a few syntax errors in your JSS.

The last child selector should be:

'&:last-child': {}

Here a complete example

const theme = createMuiTheme({
  overrides: {
    MuiTableCell: {
      root: {
        paddingTop: 4,
        paddingBottom: 4,
        "&:last-child": {
          paddingRight: 5
        }
      }
    }
  }
});