*{
        margin: 0;
        padding: 0;
        box-sizing: border-box;
        font-family: Arial, Helvetica, sans-serif;
    }
    
    body{
        display: flex;
        justify-content: center;
        align-items: center;
        height: 100vh;
        background-color: black;
    }
    
    /* full calcu div */
    .calcu{
        /* display: flex; */
        justify-content: center;
        align-items: center;
        /* height: 200px; */
        max-width: 400px;
        width: 100%;
        border-radius: 12px;
        padding: 10px 20px 20px;
        position: relative;
        background-color: lightblue;
    }
    
    /* input bar */
    .display{
        height: 40px;
        width: 80%;
        border: none;
        outline: none;
        text-align: right;
        margin-bottom: 5px;
        font-size: 20px;
        pointer-events: none;
        /* display: inline-block; */
        /* align-items: center;
        justify-content: center; */
    }
    
    /* operator buttons */
    .opt-buttons{
        display: grid;
        grid-gap: 10px;
        grid-template-columns: repeat(4, 1fr);
    }
    
    .opt-buttons button{
        padding: 10px;
        border-radius: 6px;
        border: none;
        font-size: 20px;
        cursor: pointer;
    }
    
    .opt-buttons button:active{
        transform: scale(0.99);
    }
    
    /* container: display and clear */
    .con{
        display: grid;
        grid-gap: 10px;
        grid-template-columns: repeat(2, 1fr);
    }
    
    .con button{
        padding: 10px;
        margin-bottom: 5px;
        border-radius: 6px;
        border: none;
        font-size: 20px;
        cursor: pointer;
    }
    
    .con button:active{
        transform: scale(0.99);
    }
    
    .opt, .clear{
        color: grey;
    }