style.css

body{
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background: linear-gradient(359deg, #4e2bee, #1a0868, #000000);
    background-size: 600% 600%;
    -webkit-animation: AnimationName 13s ease infinite;
    -moz-animation: AnimationName 13s ease infinite;
    -o-animation: AnimationName 13s ease infinite;
    animation: AnimationName 13s ease infinite;
}
@-webkit-keyframes AnimationName {
    0%{background-position:53% 0%}
    50%{background-position:48% 100%}
    100%{background-position:53% 0%}
}
@-moz-keyframes AnimationName {
    0%{background-position:53% 0%}
    50%{background-position:48% 100%}
    100%{background-position:53% 0%}
}
@-o-keyframes AnimationName {
    0%{background-position:53% 0%}
    50%{background-position:48% 100%}
    100%{background-position:53% 0%}
}
@keyframes AnimationName {
    0%{background-position:53% 0%}
    50%{background-position:48% 100%}
    100%{background-position:53% 0%}
}

#calculator{
    font-family: Arial, Helvetica, sans-serif;
    background-color: hsl(0, 0%, 15%);
    border-radius: 15px;
    max-width: 500px;
    overflow: hidden;
    box-shadow: 0 5px 15px rgba(0,0,0,0.75);
    border-radius: 20px;
}

#display{
    width: 100%;
    padding: 20px;
    font-size: 3rem;
    border: none;
    outline: none;
    background-color: hsl(0, 0%, 20%);
    text-align: left;
    color: white;

}

#keys{
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
    padding: 25px;

}

button{
    width: 100px;
    height: 100px;
    border-radius: 50px;
    border: none;
    background-color: hsl(0, 0%, 30%);
    color: white;
    font-size: 3rem;
    font-weight: bold;
    cursor: pointer;   
}
button:hover{
    background-color: hsl(0, 0%, 35%);
}
button:active{
    background-color: hsl(0, 0%, 40%);
}

.op_btn{
    background: linear-gradient(359deg, #4e2bee, #1a0868, #000000);
    background-size: 600% 600%;
    -webkit-animation: AnimationName 13s ease infinite;
    -moz-animation: AnimationName 13s ease infinite;
    -o-animation: AnimationName 13s ease infinite;
    animation: AnimationName 13s ease infinite;
}