*,
*::before,
*::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: monospace;
}

h1 {
  text-align: center;
  margin-bottom: 50px;
  font-size: 48px;
}

#container {
  margin: 0 auto;
  width: 406px;
  height: 610px;
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(100px, 1fr));
  grid-template-rows: repeat(auto-fit, minmax(100px, 1fr));
  gap: 2px;
}

#display {
  grid-column: 1/-1;
  background-color: darkgray;
  border-radius: 20px;
  font-size: 24px;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: end;
  padding: 20px;
  box-shadow: inset 0px -5px 10px 1px rgba(0, 0, 0, 0.3);
}

button {
  border: none;
  border-radius: 20px;
  background-color: gainsboro;
  font-size: 24px;
  box-shadow: inset 0px -5px 10px 1px rgba(0, 0, 0, 0.3);
}

button:nth-of-type(4n) {
  background-color: orange;
}

button[value='='] {
  background-color: gold;
  grid-column: 3/-1;
}