CSS Button is not displayed properly

Question

I have written the CSS code below to create a button, the result is like this: HTML CSS OUTPUT PICTURE ATTACHED HERE, there is some kind of grey button around the logout text and I only want the red background without the smaller grey thing. May I know how I can get rid of it? Thank you!

.header .logout{
  background-color: #990000;
  border: none;
  color: white;
  padding: 15px 32px;
  text-align: center;
  text-decoration: none;
  display: inline-block;
  font-size: 16px;
  margin: 4px 2px;
  cursor: pointer;
}