index.css

*{
 margin: 0;
 padding: 0;
 box-sizing: border-box;   
}
body{
    min-height: 100vh;
    background-color: #efeff2;
    display: flex;
    justify-content: center;
    align-items: center;
}
.calculator{
    width: 300px;
    height: 550px;
    box-shadow: 4px 4px 30px rgba(0,0,0,0.3);
    border-radius: 12px;
    background: #22252d;
    overflow: hidden;
}
h1{
    text-align: center;
    color: darkgrey;
    font-size: 2rem;
    font-weight: bold;
    letter-spacing: 1px;
    line-height: 55px;
}

form input{
    width: 100%;
    height: 150px;
    border: none;
    border-radius: 12px;
    font-size: 2rem;
    padding: 1rem;
    color:#fff;
    background: #000;
    text-align: right;
    pointer-events: none;
}

.btns{
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
    padding: 20px;
}
button{
    flex:0 0 22%;
    margin: 5px 0;
    border: 1px soild #000;
    width: 60px;
    height: 52px;
    font-size: 22px;
    font-weight: 600;
    border-radius: 5px;
    cursor:pointer;
}
.btn-yellow{
    background-color: rgb(245, 146,62);
    color:#fff;
}
.btn-grey{
    background: rgb(224,224,224);
}
.btn-equal{
    background: green;
}
.btn-clear{
    background: red;
}