style.css

body {
  margin: 0;
  background-color: #D14A4C;
}

.calculator {
  overflow: hidden;
  margin: 50px auto;
  width: 280px;
  height: 495px;
  background-color: white;
  border-radius: 10px;
}

section.display {
  width: 100%;
  height: 148px;
  background-color: #313748;
}

section.buttons {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-evenly;
  align-items: stretch;
  align-content: stretch;
  height: 347px;
}

section.buttons button {
  flex-grow: 1;
  flex-basis: 25%;
  -webkit-appearance: none;
  border: 1px solid white;
  border-radius: 4px;
  background-color: #F4F5F9;
  transition: all 0.3s ease-in;
  font-size: 20px;
}

section.buttons button:focus {
  outline: none;
  border: 1px solid #BBB;
}

section.buttons button:active {
  border: 1px solid #313748;
}

section.buttons button.double {
  flex-basis: 50%;
}

section.buttons button.operator {
  background-color: #DCDFE5;
}

section.buttons button.equals {
  background-color: #BD4345;
  color: white;
}