style.css

/*~~~~~ Fira Code Font Family From Google Font ~~~~~*/
@import url('https://fonts.googleapis.com/css2?family=Fira+Code:wght@300;400;500;600;700&display=swap');
/*~~~~~ Start Css Code ~~~~~*/
*, *::after, *::before {
  box-sizing: border-box;
  font-family: 'Fira Code', monospace;
}
body {
  font-size: 16px;
  line-height: 1.5;
  font-weight: normal;
}
main {
  text-align: center;
  width: 70%;
  margin: 1em auto;
  padding: 1em 0;
  border: 1px solid #EEE;
}
main span.brand {
  font-size: 3.5em;
}
main h1 {
  margin-top: 0;
  font-weight: 600;
}
main p {
  font-size: 18px;
}
main p a {
  font-weight: 700;
  color: #55BD8A;
}
/* ~~~ Calculator Code ~~~  */
.calculator-wrap {
  width: 50%;
  margin: auto;
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  grid-template-rows: minmax(60px, auto) repeat(5, 60px);
  justify-content: center;
  align-items: center;
  row-gap: 20px;
  column-gap: 10px;
}

#a_num, #b_num, #c_num {
  height: 100%;
}

input {
  outline: none;
  border: 1px solid #cecece;
  text-align: center;
  font-weight: 700;
  font-size: 1.4em;
  grid-column: span 2;
}

label strong {
  color: #55bd8a;
  font-size: 1.2em;
} 

.display-equation, .result {
  background-color: #1B1B1B;
  color: #55bd8a;
  width: 80%;
  margin: auto;
  display: grid;
  grid-template-columns: repeat(1, 1fr);
  font-size: 2em;
  font-weight: 700;
  text-align: center;
  margin-bottom: 2em;
  padding: 1em;
}

mark, #big_msg {
  text-align: center;
  background-color: #ffe60b;
  color: #212121;
  grid-column: 1 / -1;
}

#big_msg {
  background-color: #ff0102;
  color: #FFF;
  margin: 1em;
  display: block;
}

button {
  color: #1B1B1B;
  background-color: #ffe60b;
  padding: 1em;
  border: 0;
  outline: 0;
  border-radius: 2px;
  margin-top: 1em;
  cursor: pointer;
}
button:hover {
  color: #333;
  background-color: #ffc107;
}

.result {
  background-color: #FFF;
  border: 1px solid #1B1B1B;
  color: #55bd8a;
}