styles.css

:root {
  background-attachment: fixed;
  background-image: url('desk.jpeg');
  background-repeat: no-repeat;
  background-size: cover;
}

* {
  box-sizing: border-box;
}

.calc {
  border: 1px solid #ccc;
  border-radius: 5px;
  box-shadow: 4px 4px 10px 2px #3e3e3e;
  left: 50%;
  position: absolute;
  top: 50%;
  transform: translate(-50%, -50%);
  width: 400px;
}

.calc-screen {
  background-color: #252525;
  border: 1px solid #ccc;
  border-radius: 5px 5px 0 0;
  color: #fff;
  font-size: 3rem;
  height: 80px;
  margin: 2px;
  opacity: .9;
  padding-left: 10px;
  padding-right: 20px;
  text-align: right;
  width: 395px;
}

button {
  background-color: #f7f7f7;
  background-image: linear-gradient(to bottom, transparent, transparent 50%, rgb(0, 0, 0, .04));
  border: 1px solid #c4c4c4;
  border-radius: 3px;
  box-shadow: inset 0 0 0 1px rgb(255, 255, 255, .05),
              inset 0 1px 0 0 rgb(255, 255, 255, .45),
              inset 0 -1px 0 0 rgb(255, 255, 255, .15),
              0 1px 0 0 rgb(255, 255, 255, .15);
  color: #333;
  font-size: 1.25rem;
  height: 60px;
  opacity: .8;
  text-shadow: 0 1px rgb(255, 255, 255, .4);
}

button:hover {
  background-color: #e3e3e3;
}

.calc-keys {
  display: grid;
  grid-gap: 20px;
  grid-template-columns: repeat(4, 1fr);
  padding: 20px;
}

.clear-key {
  background-color: #f0595f;
  border-color: #b0353a;
  color: #fff;
}

.clear-key:hover {
  background-color: #f17377;
}

.operator {
  color: #337cac;
}

.equal-sign {
  background-color: #2e86c0;
  border-color: #337cac;
  color: #fff;
  grid-area: 6 / 3 / 6 / 5;
}

.equal-sign:hover {
  background-color: #4e9ed4;
}