The headers' border goes dark under my navigation links

I’ve tried everything including decreasing the text size but there always seems to make the border a darker shade.

*{
  font-family: Helvetica;
  background-color: black;
  color: seashell;
  opacity:0.9;
  font-size: 22px;
  box-sizing: border-box;
}
header{
  width: 100%;
  height: 69px;
  border-bottom: 1px solid seashell;
  position: fixed;
  display: flex;
  align-items: center;
  justify-content: space-between;
}
.logo{
  height: 50px;
  margin-left: 10px;

}
li{
  display: inline-block;
  text-decoration: underline;
}

<!DOCTYPE html>
<html>
<head>
  <title>The Tea Cozy</title>
  <link rel="stylesheet" href="style.css" type="text/css" />
</head>
  <body>
     
    <header>
      <img class="logo" src="https://content.codecademy.com/courses/freelance-1/unit-4/img-tea-cozy-logo.png">
      
      <nav>
        <ul>
          <li>Mission</li> <li>Featured Tea</li> <li>Locations</li>
        </ul>
      </nav>
    </header>
  </body>
</html>

the opacity:09 is decreasing the brightness of everything.

Thank you for your help. I’ve figured out that it was because I put all the characteristics under the *. so I split all the characteristics in their correct containers, and that seemed to fix the problem.