style.css

body {
  display: flex;
  justify-content: center;
  align-items: center;
  text-align: center;
  min-height: 100vh;
  font-family: sans-serif;
}
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  outline: none;
}
.main {
  width: 250px;
  background-color: #464c6b;
}
#display {
  width: 250px;
  background-color: rgb(255, 255, 255);
  outline: none;
  border: none;
  box-shadow: 1px 1px 3px black, -1px -1px 3px black;
  height: 60px;
  text-align: right;
  font-size: 20px;
  font-family: sans-serif;
  color: #000;
  max-width: 250px;
}
input[type="button"] {
  width: 35px;
  height: 35px;
  margin: 6px;
  color: #fff;
  background-color: #464c6b;
  border: none;
  font-size: 20px;
  padding: 4px;
  border-radius: 7pxx;
}
input[type="button"]:hover {
  text-shadow: 1px 1px 10px #000;
  background-color: #5be35b;
}
#clear {
  background-color: #db2612;
}
#clear:hover {
  text-shadow: none;
}
#equal {
  background-color: #5be35b;
}