#container {
    display: flex;
    flex-direction: column;
    align-items: center;
}

#target {
    background-color: black;
    border: 1px solid lightgray;

    color: lightgreen;
    font-family: 'Roboto', sans-serif;
    font-size: 90px;
    letter-spacing: -1px;

    width: 160px;
    height: 100px;
    margin: 1px;

    display: inline-flex;
    justify-content: center;
    align-items: center;
}

#clock {
    display: inline-block;
}

#whiteboard {
    width: 250px;
    height: 100px;
    margin: 10px;
    border: 1px solid black;

    background-color: #fff;
    background-size: 25px 25px;
    background-image: linear-gradient(to right, grey 1px, transparent 1px), linear-gradient(to bottom, grey 1px, transparent 1px);

    font-size: 45px;
    word-break: break-all;
}

#buttons-container {
    display: grid;
    grid-template-columns: 80% 20%;
    grid-template-rows: 16% 28% 28% 28%;
    grid-template-areas: "clear clear"
        "numbers operations"
        "numbers operations"
        "numbers operations";
    justify-items: center;
    width: 300px;
}

#start {
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 60px;
    letter-spacing: 4px;
    border: 1px solid black;
    margin: 1px;
    padding: 5px;
    grid-area: clear;
    cursor: pointer;
}

#numbers {
    grid-area: numbers;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
}

#operations {
    grid-area: operations;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
}

.number {
    background-color: blue;
    border: 1px solid black;

    color: white;
    font-family: 'Roboto', sans-serif;
    font-size: 70px;
    letter-spacing: -1px;

    width: 110px;
    height: 110px;
    margin: 1px;

    display: inline-flex;
    justify-content: center;
    align-items: center;

    cursor: pointer;
}

.number.active {
    background-color: gray;
    color: black;
}

.operation {
    background-color: lightblue;
    border: 1px solid black;

    color: white;
    font-family: 'Roboto', sans-serif;
    font-size: 35px;

    width: 50px;
    height: 50px;
    margin: 1px;

    display: inline-flex;
    justify-content: center;
    align-items: center;

    cursor: pointer;
}