*{
    padding: 0;
    margin: 0;
    box-sizing: border-box;
    font-family: century; 
}
header{
    background-image: linear-gradient(rgba(0, 0, 0, 0.6),rgba(0, 0, 0, 0.6)), url(./image/images\ \(7\).jpeg);
    height: 100vh;
    background-size: cover;
    background-position: center;
}
.arithmetique {
    color: #ffffff;
    padding: 0px 10px;
    border: solid transparent;
    position: absolute;
    top: 10px;
    left: 0px;
    text-transform: uppercase;
    text-decoration: none;
    font-weight: 500;
    text-shadow: #000000b3;
}
.arithmetique span{
    color: rgb(238, 56, 11);
    font-weight:bold;
    font-size: 1.5rem;
}
ul{
    list-style: none;
    float: right;
    display: flex;
    margin-top: 20px;
}
ul li a{
    text-decoration: none;
    color: #fff;
    padding: 0px 20px;
    text-transform: uppercase;
    border: solid transparent;
    transition: 1s;
    font-weight: 500px;
    margin : 10px
}
ul li a:hover{
    background: #fff;
    color: #000;
}
ul li.active a{
    background: #fff;
    color: #000;
}
.principale{
    max-width: 114rem;
    margin: 0 auto;
}
.titre{
    position: absolute;
    top: 70%;
    left: 50%;
    transform: translate(-50%, -50%);
}
.titre h1{
    color: #fff;
    font-size: 70px;
}
.bouton{
    position: absolute;
    top: 80%;
    left: 50%;
    transform: translate(-50%, -50%);
}
.bouton a{
    color: #fff;
    text-decoration: none;
    border: 1px solid #fff;
    padding: 10px 20px;
}
.btn{
    transition: 1s;
}
.btn:hover{
    background-color: #fff;
    color: #000;
}
.rien{
    position: absolute;
    top: 40%;
    left: 50%;
    transform: translate(-50%, -50%);
}