How can I extend the header to the full page width? [duplicate]

How can I extend my header to the full page? I have tried margin-left & right but that doesn't work.

Header.css

.header{
    background: green;
    height: 70px;
    width: 100%;
    display: flex;
    justify-content: space-between;
    margin-bottom: 25px;
    left: 0;
    right: 0;
} 


  .header-right {
    float: right;
  }

  @media screen and (max-width: 500px) {
    .header-right {
      float: none;
    }
  }

Here's my app.tsx file:

const Header = () => (
  <div className = 'header'>
  <h1>Instaride</h1>
  <div className="header-right">
    <Button> Sign In</Button>
    <Button> Contact</Button>
  </div>
</div>
);

export default Header;

I have already tried this too:

body, html{
    margin:0;
    padding:0;
}