active-scroll-navigation/demo/css/app.css

Navigation will change active state when user scroll.

body{
  font-family: 'Source Sans Pro', sans-serif;
  font-weight: 300;
  padding: 0;
  margin: 0;
  font-size: 18px;
}

nav{
  position: fixed;
  background: rgba(0,0,0, .2);
  width: 100%;
  top:0;
  padding: 10px 0;
  display: flex;
  justify-content: center;
  z-index: 99;
}
nav a{
  margin: 0 15px;
  text-decoration: none;
  color: #000;
  text-transform: uppercase;
  font-weight: 400;
  font-size: 20px;
}
nav a.main-nav__item--active{
  color: #4286f4;
}
section{
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
}

#section1,
#section3{
  background: #e5e6e8;
}