Gin Theme Menu CSS Drupal 9- Horizontal Menu

Create Menu in Structure.

Place menu as a block where you need it and configure it in "Blocks Layout"

Here's the CSS: (Note: I had to save the css directly to the Gin styles.css for it to work. It would not work in my custom theme)

#block-YourMenuMachineName > ul{
  display: flex;
  justify-content: center;
  list-style-type: none;
  margin-top: 50px;
  margin-bottom: 25px;
  padding: 0;
  overflow: hidden;
  background-color: #333333;
  width: 100%;

}

#block-YourMenuMachineName > ul > li{
flex: 1;

}

#block-YourMenuMachineName> ul > li > a{
  font-size: 18px;
  display: block;
  color: #ff0099;
  text-align: center;
  padding: 10px;
  text-decoration: none;

}

#block-YourMenuMachineName > ul > li > a:hover{
  background-color: #3279a8;

}

#block-YourMenuMachineName li a:active{
  background-color: #fff;

}