style.css

:root {
    font-family: Arial, Helvetica, sans-serif;
    font-size: 16px;
}

[hidden] {
    display: none !important;
}

h1 {
    font-size: 3rem;
    text-align: center;
    margin: 0.5rem
}
    h1 span {
        margin-left: 0.5rem;
        font-weight: normal;
        font-size: 0.5em;
    }

.root-flex-con {
    display: flex;
    flex-direction: column;
}

.root-flex-overlay-con {
    display: flex;
    flex-direction: column;
    position: absolute;
    top: 0;
    left: 0;
    z-index: 0;

    margin: 0.5rem
}

.selection {
    margin: 1rem;
    display: flex;
    flex-direction: row;
    justify-content: center;
}
    .selection h2 {
        margin: 0.5rem
    }
.selection-box {
    border-style: solid;
    border-radius: 1rem;
    border-color: #000000;
    border-width: 0.2rem;

    margin-right: 0.5rem;

    display: flex;
    flex-direction: row;

    width: min-content;
    height: min-content;

    align-self: center;
}
    .selection-box select {
        margin: 0rem;
        background-color: #00000000;
        border: none;

        font-size: 1rem;
        text-align: center;
    }

.important-button {
    font-size: 2rem;
    font-weight: bold;

    cursor: pointer;

    background-color: #00000000;
    align-self: center;
    width: min-content;

    border-style: solid;
    border-radius: 1rem;
    border-color: #000000;
    border-width: 0.2rem;

    margin: 1rem;
}
.important-button:active {
    box-shadow: 0 0 0.2rem 0.1rem;
}

.calendar {
    display: flex;
    flex-direction: row;
    flex-wrap: wrap;
    justify-content: center;

    margin: 1rem;
}
    .calendar .month {
        display: flex;
        flex-direction: column;
        align-items: center;
    }
    .calendar h3 {
        margin: 0.2rem

    }
        .calendar .month .weeks {
            border-top: 0.15rem solid #000000;
            width: 12rem;
            margin: 0.5rem;
        }
        .calendar .month .weeks th {
            font-size: 0.5rem;
        }
        .calendar .month .weeks td {
            text-align: center;
            font-size: 0.9rem;
            font-weight: bold;
        }
        .calendar .month .weeks td:nth-last-child(1), 
        .calendar .month .weeks td:nth-last-child(2),
        .calendar .month .weeks th:nth-last-child(1), 
        .calendar .month .weeks th:nth-last-child(2) {
            color: #525252
        }