* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

main {
    display: flex;
    flex-direction: column;
    width: 100vw;
    height: 100vh;
    align-items: center;
    justify-content: center;
    background-color: #ADD8E6;
}

h1 {
    font-family: Arial, Helvetica, sans-serif;
    margin-bottom: 10px;
    text-transform: uppercase;
}

nav {
    display: flex;
    flex-direction: column;
    border-radius: 8px;
    text-align: center;
    border: 2px solid black;
    padding: 10px;
    background-color: rgb(61, 60, 60);

}

.displayScreen {
    display: flex;   
    flex-direction: column;
    gap: 5px;
    text-align: center;
}

#displayExpression {
    height: 70px;
    border-radius: 8px;
    text-align: center;
    font-size: 30px;
}

.containerButtons {
    display: flex;
    flex-direction: column;
    height: 400px;
    margin-top: 10px;
    align-items: center;
}

.btOperButtons {
    margin-top: 05px;
    width: 90px;
    height: 60px;
    border-radius: 8px;
    background-color: rgb(90, 89, 89);
    color: white;
    border: none;
    font-size: 25px;
}

.btOperButtons:hover {
    background-color: rgb(38, 38, 39);
    cursor: pointer;
}

#btNumComma {
    text-align: center;
}

#btOperEquality {
    background-color: rgb(48, 102, 190);
}

#btOperEquality:hover {
    background-color: blue;
}

#infoFooter {
    font-size: 20px;
    font-style: italic;
}

footer {
  position: fixed;
  bottom: 0;
  left: 0;
  width: 100%;
  background-color: #333;
  color: white;
  padding: 10px;
  text-align: center;
  z-index: 1000;
  font: bold 12pt Arial;
}

footer > p > a {
    color: white;
}

#btNothing {
    visibility: hidden;

}

@media (max-width: 750px) {
    h1 {
        font: bold 20pt arial black;
        margin-bottom: 10px;
    }

    nav {
        height: 350px;
    }

    .displayScreen {
        align-items: center;
    }

    #displayExpression {
        height: 40px;
        width: 255px;
        font-size: 18px;
    }

    .containerButtons {
        display: flex;
        flex-direction: column;
    }

    .btOperButtons {
        width: 60px;
        height: 40px;
        font-size: 18px;
    }

    .btOperButtons:hover {
        background-color: rgb(90, 89, 89);
    }

    #infoFooter {
        font-size: 16px;
    }
}

@media (max-width: 350px) {
    h1 {
        font: bold 15pt arial black;
        margin-bottom: 10px;
    }

    nav {
        height: 280px;
    }

    .displayScreen {
        align-items: center;
    }

    #displayExpression {
        height: 35px;
        width: 215px;
        font-size: 15px;
    }

    .containerButtons {
        display: flex;
        flex-direction: column;
    }

    .btOperButtons {
        width: 50px;
        height: 30px;
        font-size: 15px;
    }

    .btOperButtons:hover {
        background-color: rgb(90, 89, 89);
    }

    #infoFooter {
        font-size: 14px;
    }
}

@media (max-width: 240px) {
    h1 {
        font: bold 10pt arial black;
        margin-bottom: 8px;
    }

    nav {
        height: 160px;
        width: 136px;
    }

    .displayScreen {
        align-items: center;
    }

    #displayExpression {
        height: 15px;
        width: 118px;
        font-size: 10px;
        border-radius: 6px;
    }

    .containerButtons {
        display: flex;
        flex-direction: column;
    }

    .btOperButtons {
        width: 25px;
        height: 10px;
        font-size: 8px;
    }

    .btOperButtons:hover {
        background-color: rgb(90, 89, 89);
    }

    #infoFooter {
        font-size: 10px;
    }
}

@media (max-width: 140px) {
    body {
        background-color: #ADD8E6;
    }

    h1 {
        font: bold 8pt arial black;
        margin-bottom: 3px;
    }

    nav {
        height: 150px;
        width: 100px;
    }

    .displayScreen {
        align-items: center;
    }

    #displayExpression {
        height: 10px;
        width: 80px;
        font-size: 5px;
        border-radius: 4px;
    }

    .containerButtons {
        display: flex;
        flex-direction: column;
    }

    .btOperButtons {
        width: 15px;
        height: 8px;
        font-size: 6px;
    }

    .btOperButtons:hover {
        background-color: rgb(90, 89, 89);
    }

    #infoFooter {
        font-size: 5px;
    }
}