style.css

html, body {
    margin: 0;
    padding: 0;
}

.calendar {
    border: 2px solid black;
    border-spacing: 5px;
    width: 100%;
    height: 100vh;
}

.calendar td {
    border: 1px dashed gray;
    padding: 5px;
    margin: 5px;
}

.calendar-buttons {
    text-align: center;
    cursor: pointer;
}

.calendar-inputs {
    height: 100%;
    width: 40%;
    text-align: center;
}

#calendarGoToButton {
    width: 15%;
    height: 100%;
}

.calendar-day-title {
    font-size: 2em;
    text-align: center;
}

.calendar-day {
    position: relative;
    cursor: pointer;
}

.calendar-day-label {
    position: absolute;
    top: 0;
    left: 0;
}

.today {
    background: yellow;
}

.popup {
    border: 10px solid yellow;
    border-radius: 20px;
    width: 50%;
    position: absolute;
    top: 10px;
    cursor: pointer;
    background: white;
    text-align: center;
    
    /* за да е центрирано */
    margin-left: auto;
    margin-right: auto;
    left: 0;
    right: 0;
}

.popup-hidden {
    display: none;
}

.display-none {
    display: none;
}

.display-block {
    display: block;
}