.header {
  background: pink;
  padding: 10px;
  margin: 10px;
  border: 10px solid aquamarine;
}

.sidebar{
 max-width: 25%;
 background: linear-gradient(#dcf3f5, grey);
 border: 10px dotted;
 display: inline-block;
}

.main{
  background: bisque;
  border: 10px solid black;
  display: inline-block;
}