:root {
    --primary : #ff0185;
    --secondary : #091126;
    --basic-bg: linear-gradient(45deg, #047df4, #01188b);
    --standard-bg: linear-gradient(45deg, #00ff62, #233543);
    --premium-bg: linear-gradient(45deg, #ff2ff7, #4700ff);
    --primary-color: #111d29;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: sans-serif;
    scroll-behavior: smooth;
}

body {
    display: flex;
    flex-direction: column;
    background-color: var(--primary-color);
    overflow: overlay;
    background: linear-gradient(to top, var(--primary-color), transparent, transparent),
    url(maths.jpg) top / cover fixed;
    background-repeat: no-repeat;
}

body::-webkit-scrollbar {
    width: .5rem;
}

body::-webkit-scrollbar-thumb {
    width: .5rem;
    background-color: magenta;
    border-radius: .5rem;
}

header {
    position: sticky;
    top: 0;
    background-image: linear-gradient(45deg, navy, magenta 120%);
    color: #fff;
    transition: .5s ease-in-out;
    z-index: 500000;
}

header:hover {
    transform: translateY(0) !important;
}

header nav {
    height: 3.7rem;
    display: flex;
    justify-content: space-around;
    align-items: center;
}

header nav .logo {
    color: #fff;
    font-size: 1.4rem;
    text-decoration: none;
}

header nav .logo strong {
    color: cyan;
}

header nav ul {
    list-style: none;
    display: flex;
}

header nav ul li {
    padding: 1rem 1.5rem;
    border-radius: .6rem;
}

header nav ul li:hover {
    cursor: pointer;
    background-color: #fff4;
}

li {
    color: whitesmoke;
}

.actif {
    color: cyan;
}

@media screen and (max-width: 650px) {
    header {
        bottom: 0;
        order: 2;
    }
    main {
        order: 1;
    }
}

.center {
    display: block;
    margin-left: auto;
    margin-right: auto;
    width: 10%;
  }