.container {
  width: 100%;
  margin: 0 auto;
  display: flex;
  justify-content: space-between;
  flex-direction: row;
}
.side-menu {
  width: 23%;
  padding: 10px;
  background-color: #faa;

}

.main-contents {
  width: 73%;
  padding: 10px;
  background-color: #ccc;
}
.side-menu, .main-contents {
  box-sizing: border-box;
}