layout.css

html, body, div, p, ul, li {
  margin: 0;
  padding: 0;
  text-align: center;
  color: white;
}

ul {
  list-style: none;
}

body {
  height: 100%;
  display: flex;
  justify-content: center; 
  -webkit-justify-content: center;
  align-items: center; 
  -webkit-align-items: center;
}


.container {
  display: flex;
  display: -webkit-flex;
  display: -moz-flex;
  display: -ms-flex;
  justify-content: center; 
  -webkit-justify-content: center;
  align-items: center; 
  -webkit-align-items: center;
  flex-wrap: wrap;
  width: 400px;
  height: 400px;
}

.row {
  display: flex;
  display: -webkit-flex;
  display: -moz-flex;
  display: -ms-flex;
  flex-grow: 1;
  -webkit-flex-grow: 1;
  -webkit-flex-grow: 1;
  -moz-flex-grow: 1;
  width: 360px;
  margin-left: 20px;
  margin-right: 20px; 
  vertical-align: middle;
} 

.display {
  width: 100%;
  height: 40px;
  margin: 0 10px;
  padding: 0 10px 0 30px;
  background-color: lightslategray;
  text-align: right;
  font-size: x-large;
}

.number-key {
  width: 70px;
  margin: 10px;
  background-color: orange;
  cursor: pointer;
}

.operator-key {
  width: 70px;
  margin: 10px;
  background-color: grey;
  cursor: pointer;
}

.equal-key {
  width: 70px;
  margin: 10px;
  background-color: grey;
  cursor: pointer;
}

.function-key {
  width: 70px;
  margin: 10px;
  background-color:  red;
  cursor: pointer;
}

.number-key:hover, .operator-key:hover, .function-key:hover {
  box-shadow: 3px 3px 3px #888888
}