style.css

body {
  font-family: 'Anton', sans-serif;
}

.calculator {
  width: 300px;
  display: grid;
  grid-template-columns: 1fr 1fr 1fr 1fr;  
}

#display {
  grid-column: 1 / span 4;
  background-color: #eee;
  height: 75px;
  border: 2px solid #222;
  text-align: right;
  font-size: 3.5rem;
  padding-right: 5px;
}

.op {
  background-color: #ccc;
  height: 75px;
  border: 1px solid #111;  
  text-align: center;
  font-size: 2rem;
}

.op:hover { 
  background-color: rgb(255, 255, 213);
  transition-duration: 500ms;
}

.op p {
  margin-top: 50%;
  transform: translate(0, -50%);
  line-height: 0%;
}

.figure {
  background-color: #ddd;
  height: 75px;
  border: 1px solid #111;
  text-align: center;
  font-size: 2rem;
}

.figure:hover { 
  background-color: rgb(224, 224, 255);
  transition-duration: 500ms;
}

.figure p {
  margin-top: 50%;
  transform: translate(0, -50%);
}

#zero {
  grid-column: 2 / span 2;
  display: inline;
}

#zero p {
  margin-top: 25%;
}

#displaySecondary {
  grid-column: 1 / span 4;
  
  display: inline-grid;
  grid-template-columns: 3fr 25px 1fr;
  background-color: #eee;
  height: 30px;
  border: 2px solid #222;
}

#currentOperation {
  margin: 2px;
  border-radius: 50%;
  align-content: center;
  justify-content: center;
  text-align: center;
}

#resultFlag {
  margin: 2px;
  border-radius: 4px;
  align-content: center;
  justify-content: center;
  text-align: center;
}