style.css

html {
  box-sizing: border-box;
}
body {
    margin: 0;
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    background: linear-gradient(to right, #076585, #fff);
}
.calculator{
  width: 400px;
  background-color: whitesmoke;
  border-radius: 12px;
  box-shadow: 0 5px 30px -5px rgba(206, 118, 118, 0.6);
}
/* Display */
.calculator-display{
  background-color: black;
  color: white;
  display: flex;
  align-items: center;
  justify-content: flex-end;
  border-radius: 12px 12px 0 0;
}
.calculator-display h1{
  margin: 0;
  padding: 25px;
  font-size: 45px;
  font-family:'Times New Roman', Times, serif;
  font-weight: 100;
  overflow-x:auto;
}

/* Custom Scrollbar */
/* width */
::-webkit-scrollbar {
  width: 10px;
}

/* Track */
::-webkit-scrollbar-track {
  background: #f1f1f1;
}

/* Handle */
::-webkit-scrollbar-thumb {
  background: rgb(53, 42, 42);
}

/* Handle on hover */
::-webkit-scrollbar-thumb:hover {
  background: rgb(201, 8, 8);
}
 
.calculator-buttons{
  display: grid;
  grid-template-columns: repeat(4, 1fr); 
  grid-gap: 10px;
  
}
button{
  min-height: 50px;
  font-size: 30px;
  font-weight: 100;
  border: none; 
  color: white;
  background-color: gray;
  margin: 5px;
  border-radius: 10px; 
  cursor: pointer;
}
.red{
  background-color: red !important;
  color: #000;
}
.operator{
  background-color: rgb(97, 105, 97);
  color: rgb(216, 255, 144);
  border: none;
}
#clear-btn{
  background-color: rgb(211, 85, 85);
  color: rgb(216, 255, 144);
}
button:hover{
  filter: brightness(110%);
}
button:active {
  transform: translateY(2px);
}
button:focus {
  outline: red;
}
.equal {
  background-color: rgb(154, 196, 154);
  color: white; 
  grid-column: -2;
  grid-row: 2 / span 4 ;
}

/* Media Query: Large Smartphone (Vertical) */
@media only screen and (max-width: 600px){
  .calculator{
    width: 95%;
  }
}