loginForm.css

.ui.form.formStyle  {
    margin-left: 300px ; 
    margin-top: 100px ;
    width: 550px ;
    height: 400px;
}

.ui.button.buttonStyle {
    border-radius: 18px;
    width: 200px;
    background-color:#3865FE;
}

loginForm.js

import React from "react";
import { Button, Form, Header } from "semantic-ui-react";
import './loginForm.css';

const LoginForm = () => (
  <Form className='formStyle'>
    <Button className='buttonStyle'>Login</Button>
  </Form>
);

export default LoginForm;