<div class="cp_navi03">
<ul class="sidenav">
  <li><a href="#home" class="active">Home</a></li>
  <li><a href="#news">News</a></li>
  <li><a href="#contact">Contact</a></li>
  <li><a href="#about">About</a></li>
</ul>
</div>

.cp_navi03 ul.sidenav {
  position: relative;
  display: flex;
  flex-direction: column;
  width: 200px;
  height: auto;
  margin: 0;
  padding: 0;
  background: #f1f1f1;
  list-style-type: none;
}
.cp_navi03 ul.sidenav li a {
  display: block;
  color: #000000;
  padding: 15px;
  text-decoration: none;
}
.cp_navi03 ul.sidenav li a.active {
  background: #da3c41;
  color: #ffffff;
}
/*.active以外のhoverの動き*/
.cp_navi03 ul.sidenav li a:hover:not(.active) {
  background-color: #1b2538;
  color: #ffffff;
}
@media screen and (max-width: 900px) {
  .cp_navi03 ul.sidenav {
    flex-direction: row;
    width: 100%;
  }
}
@media screen and (max-width: 480px) {
  .cp_navi03 ul.sidenav {
    flex-direction: column;
    text-align: center;
  }
}