@media screen and (min-width: 320px) and (max-width: 640px){
  .header-description {
    max-width: 90%;
    margin: 1.5em auto 0;
    font-size: 1.3em;
    color: #b3e6ff;
  }
  .calc-display__digits-top {
    font-size: 1.3em;
  }
  .calc-button {
    font-size: 1.4em;
    max-height: 30px;
    padding: 10px 0 8px;
    margin: 0.22em;
  }
  .calc-button__operators {
    font-weight: 300;
  }
  .calc-button__horizon {
    width: 112px;
    height: 30px;
  }
  .calc-button__vertical {
    padding: 41px 0;
  }
  .calc-button:nth-last-child(1) {
    margin-top: -53px;
  }
  .calc-button:nth-last-child(2) {
    margin-top: -53px;
    margin-left: -55px;
  }
  .footer-copyright {
    font-size: 1.3em;
  }
  .fa.fa-lg {
    margin: 0 0.1em;
    font-size: 1.8em;
  }
}