* {
  box-sizing: border-box;
}
.menu {
  float: left;
  width: 20%;
}
.menuitem {
  padding: 8px;
  margin-top: 7px;
  border-bottom: 1px solid #f1f1f1;
}
.main {
  float: left;
  width: 60%;
  padding: 0 20px;
  overflow: hidden;
}
.right {
  background-color: lightblue;
  float: left;
  width: 20%;
  padding: 10px 15px;
  margin-top: 7px;
}

@media only screen and (max-width:800px) {
  /* For tablets: */
  .main {
    width: 80%;
    padding: 0;
  }
  .right {
    width: 100%;
  }
}
@media only screen and (max-width:500px) {
  /* For mobile phones: */
  .menu, .main, .right {
    width: 100%;
  }
}

      .button {
        display: inline-block;
        padding: 10px 20px;
        text-align: center;
        text-decoration: none;
        color: #ffffff;
        background-color: #7aa8b7;
        border-radius: 6px;
        outline: none;
        transition: 0.3s;
      }
      .button:hover {
        background-color: #c2c7c7;
      }