body {
  margin: 0;
  padding: 0;
  font-family: Arial, sans-serif;
  background-color: #f5f5f5;
}

.calculator {
  width: 90%;
  max-width: 600px;
  margin: 50px auto;
  background-color: #e6e6e6;
  box-shadow: 2px 2px 5px rgba(0, 0, 0, 0.2);
  border-radius: 10px;
  overflow: hidden;
}

.screen {
  background-color: #fff;
  padding: 10px;
  border-bottom: 1px solid #ccc;
}

.screen input[type="text"] {
  width: 100%;
  height: 50px;
  border: none;
  font-size: 2em;
  text-align: right;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  grid-gap: 5px;
  padding: 10px;
}

.buttons button {
  display: block;
  width: 100%;
  height: 50px;
  border: none;
  border-radius: 5px;
  background-color: #fff;
  font-size: 1.5em;
  color: #444;
  cursor: pointer;
  transition: all 0.2s;
  box-shadow: -1px 1px 3px darkgray;
}

.buttons button:hover {
  background-color: #ccc;
  color: #fff;
  box-shadow: -1px 1px 3px gray;
}

.buttons button:active {
	transition: all 0s;
	outline-color: whitesmoke;
	outline-style: ridge;
	outline-width: 1px;
	outline-offset: 1px;
	background-color: lightgrey;		
}

.buttons .clear {
  background-color: salmon;
  color: #fff;
}

.buttons .backspace {
  background: darkgrey;
  color: white;
}

.buttons .operator {
  background-color: #EEE;
  color: black;
}

.buttons .calculate {
  background-color: steelblue;
  color: #fff;
}

@media screen and (max-width: 600px) {
  .buttons {
    grid-template-columns: repeat(3, 1fr);
  }
}