style.css

body {
    background-color: #15414d;
    text-align: center;
}
.main_heading{
    color: rgb(255, 251, 5);

}
#intro{
    color: #56820a;

}
.intro2{
  color: aliceblue;
  text-align: center;
}
.intro_image{
    width: 15%;
    
}
.ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
  width: 125px;
  background-color: #555;
  position: fixed;
  height: 100%;
  overflow: auto;
}

.li a {
  margin-bottom: 38px;
  display: block;
  color: #000;
  padding: 8px 16px;
  text-decoration: sans;
  border-bottom:#ffdc00 ;
  text-align: center;
}

.li a.active {
  background-color: #4CAF50;
  color: white;
}

.li a:hover:not(.active) {
  background-color: #555;
  color: white;
}
li a.active {
  background-color: #4CAF50;
  color: white;
}

li a:hover:not(.active) {
  background-color: #555;
   color: white;
}