How to style material-ui textfield

You need to add the InputProps property to the TextField.

const styles = theme => ({
    textField: {
        width: '90%',
        marginLeft: 'auto',
        marginRight: 'auto',            
        paddingBottom: 0,
        marginTop: 0,
        fontWeight: 500
    },
    input: {
        color: 'white'
    }
});

JSX:

<TextField
    id="email"
    label="Email"
    className={classes.textField}
    value={this.state.form_email}
    onChange={this.handle_change('form_email')}
    margin="normal"
    InputProps={{
        className: classes.input,
    }}
/>

As an aside, you can also style the label or use an override as described here.