style.css

@import url('https://fonts.googleapis.com/css2?family=Poppins:ital,wght@0,100;0,200;0,300;0,400;0,500;0,600;0,700;0,800;0,900;1,100;1,200;1,300;1,400;1,500;1,600;1,700;1,800;1,900&display=swap');

*{
    margin: 0;
    padding: 0;
    font-family: "Poppins", sans-serif;
    box-sizing: border-box;
}

.container{
    display: flex;
    justify-content: center;
    align-items: center;
    background : rgb(194, 242, 244);
    width: 100%;
    height: 100vh;
}

.calc{
    width: 400px;
    border: 2px solid #00000079;
    border-radius: 10px;
    box-shadow: 0 0 20px rgba(0, 0, 0, 0.1);
    overflow: hidden;
}

h2{
    font-size: 30px;
    text-align: center;
    background-color: rgb(0, 0, 0);
    color: white;
    font-weight: 500;
    border-bottom: 5px solid #0033ff47;
    padding: 15px;
}

.display{
    background: #222;
    color: #fff;
    text-align: right;
    font-size: 35px;
    padding: 20px;
    border-bottom: 1px solid #ccc;
}

.buttons{
    display: grid;
    grid-template-columns: repeat(4, 1fr);
}

.btn{
    background: #fff;
    color: #222;
    cursor: pointer;
    padding: 20px;
    font-size: 20px;
    transition: background-color .2s;
    font-weight: 500;
}

.btn:hover{
    background-color: #d5d5d5;
}

.equal-btn{
    grid-column: span 2;
    background-color: #007bff;
    color: #fff;
}

.equal-btn:hover{
    background-color: #005fc4;
}

.num {
    background-color: #4bc9f7;
}

.op{
    background-color: #001eff;
    color: #fff;
}

.op:hover{
    color: black;
}

.bt {
    background-color: #ffa600;
}