styles.css

* {
  padding: 0;
  margin: 0;
}
body {
  width: 100vw;
  height: 100vh;
  overflow: hidden;
  display: flex;
  justify-content: center;
  align-items: center;
  background-color: #222831;
  font-family: sans-serif;
}
.calculator {
  width: 300px;
  padding-bottom: 12px;
  border-radius: 9px;
  background-color: #000;
  box-shadow: 7px 10px 10px -2px rgba(0, 0, 0, 0.61);
}
.display {
  width: 100%;
  height: 80px;
  border: none;
  box-sizing: border-box;
  padding: 10px;
  font-size: 2rem;
  background-color: #00ff44;
  color: #001;
  text-align: right;
  border-top-left-radius: 7px;
  border-top-right-radius: 7px;
}
.row {
  display: flex;
  justify-content: space-between;
}
button {
  width: 50px;
  height: 50px;
  border-radius: 50%;
  border: none;
  outline: none;
  font-size: 1.5rem;
  background-color: #222831;
  color: #00ff44;
  margin: 10px;
}
button:hover {
  cursor: pointer;
}
.operator {
  background-color: yellow;
  color: #000;
}