style.css

@import url("https://fonts.googleapis.com/css2?family=Poppins:wght@400;500;600;700&display=swap");

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

body {
    display: flex;
    flex-direction: column;
    height: 100vh;
    font-family: Poppins, sans-serif;
    background: linear-gradient(to bottom, #ff5e88, #ff9966);
}

h1 {
    font-weight: 600;
    text-align: center;
    color: white;
    margin-block: 2rem;
}

h3 {
    font-weight: 600;
}

.calendar {
    margin: 0 auto;
    width: clamp(320px, 400px, 90%);
    padding: 1rem;
    background: white;
    border-radius: 10px;
    box-shadow: 0 0 10px rgba(0, 0, 0, 0.2);
}

.calendar header {
    display: flex;
    align-items: center;
    justify-content: space-between;
}

.calendar nav {
    display: flex;
    align-items: center;
}

.calendar ul {
    list-style: none;
    display: flex;
    flex-wrap: wrap;
    text-align: center;
}

.calendar ul li {
    width: calc(100% / 7);
    position: relative;
    z-index: 2;
    aspect-ratio: 1;
    display: flex;
    align-items: center;
    justify-content: center;
}

#prev, #next {
    width: 20px;
    height: 20px;
    position: relative;
    border: none;
    background: transparent;
    cursor: pointer;
}

#prev::before, #next::before {
    content: '';
    width: 50%;
    height: 50%;
    position: absolute;
    top: 50%;
    left: 50%;
    border-style: solid;
    border-width: 0.25em 0.25em 0 0;
    border-color: #ccc;
}

#next::before {
    transform: translate(-50%, -50%) rotate(45deg);
}

#prev::before {
    transform: translate(-50%, -50%) rotate(-135deg);
}

#prev:hover::before, #next:hover::before {
    border-color: black;
}

.days {
    font-weight: 600;
}

.dates li.today {
    color: white;
}

.dates li.today::before {
    content: '';
    width: 2rem;
    height: 2rem;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    background: black;
    border-radius: 50%;
    z-index: -1;
}

.dates li.inactive {
    color: #ccc;
}