a, a:visited {
-webkit-transition: 0.5s ease;
transition: 0.5s ease; 
}

a:hover, a:visited:hover {
color: #90f431 !important;
-webkit-transition: 0s ease;
transition: 0s ease; 
}

html {
  scroll-padding-top: 15vh; /* высота вашего хедера */
}