body{
    margin: 0;
    min-height: 100vh;

    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;  

    background:
    linear-gradient(rgba(0,0,0,0.6), rgba(0,0,0,0.6)),
    url("image2.jpg");
    background-size: cover;
    background-position: center;
    background-repeat: no-repeat;
}

#heading{
    margin-bottom: 20px;     
    color: rgb(231, 231, 231);
    font-weight: bold;
    text-align: center;      
}

#calculator{
    font-family: Arial, Helvetica, sans-serif;
    background-color: rgb(10, 10, 10);

    width: 300px;             
    max-width: 320px;        

    border-top-left-radius: 5px;
    border-top-right-radius: 5px;

    border: 2px solid rgb(216, 215, 215);
    
}

#display{
    width: 100%;            
    padding: 12px;
    background-color: rgb(66, 133, 172);
    border-radius: 5px;
    font-size: 2rem;
    border: 2px solid black;
    height: 60px;
    box-sizing: border-box;  
}

#keys{
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 12px;

    padding: 15px;
    background-color: rgb(31, 29, 29);

    width: 100%;             
    max-width: 320px;       

    border-bottom-left-radius: 5px;
    border-bottom-right-radius: 5px;

    border: 2px solid rgb(39, 38, 38);
    border-top: none;
    box-sizing: border-box;
}

button{
    width: 75%;            
    height: 45px;          

    border-radius: 50%;
    background-color: rgb(172, 118, 38);

    font-weight: bold;
    font-size: 1.1rem;
    color: white;

    border: none;
    cursor: pointer;
}

button:hover{
    background-color: rgb(66, 66, 65);
    transition: 200ms;
}


@media (max-width: 480px){
    #display{
        font-size: 1.6rem;  
        height: 55px;
    }

    button{
        height: 45px;        
        font-size: 1rem;
    }
}