How do I add borderTop to React Material UI Table?

You forgot to define a borderStyle property

const styles = theme => {
    root : { borderTopWidth: 1, borderColor: 'red',borderStyle: 'solid'} // or borderTop: '1px solid red'
}

...

class TableComponent ...

{ classes } = this.props; 

<Table className={classes.root}>
</Table

export default withStyles(styles)(TableComponent)

or you can just add inline style like

<Table style={{borderTop: '1px solid red'}}>
</Table>