main.css

html, body {
    
 -webkit-user-select: none; /* Safari */        
 -moz-user-select: none; /* Firefox */
 -ms-user-select: none; /* IE10+/Edge */
  user-select: none; /* Standard */
    
    height: 100%;
    width:100%;
}

body {
    font-family: monospace;
    background-image: linear-gradient(to left, lightpink, lightblue);
}

h1 {
    padding: 25px;
    margin: 20px;
    text-align: center;
}

.container {
    justify-content: center;
    align-items: center;
    margin: auto;
    height: 410px;
    width: 360px;
    box-shadow: 6px 8px 2px grey;
    background: white;
    border-radius: 10px;
}

.output {
    position: relative;
    top: 20px;
    text-align: center;
}

input {
    text-align: end;
    padding: 8px;
    font-size: 25px;
}

#display {
    outline: none;
    width: 280px;
    height: 40px;
    background: #f1f1f1;
    border-radius: 10px;
    border-color: none;
    border: none;
}

.keyboard {
    width: 330px;
    position: relative;
    top: 40px;
    margin: 10px;
    text-align: center;
}

button {
    font-family: sans-serif;
    font-weight: bolder;
    font-size: 22px;
    color"black";
    background: white;
    border: none;
    margin: 6px;
    padding: 12px;
    outline: none;
    border-radius: 50%;
    width: 49px;
    height: 45px;
}

button:nth-child(1) {
    background: lightgrey;
}

button:nth-child(2) {
    font-weight: 550;
    background: lightgrey;
}

button:nth-child(3) {
    background: lightgrey;
}

button:nth-child(4) {
    background: lightgreen;
}

button:nth-child(8) {
    background: aqua;
}

button:nth-child(12) {
    padding: 0px;
    font-size: 30px;
    font-family: monospace;
    background: orange;
}

button:nth-child(16) {
    background: pink;
    font-size: 25px;
}

button:nth-child(20) {
    background: yellow;
}

button:hover {
    background: rgba(230, 108, 230, 0.616);
}