style.css

/* Fonts from Google Fonts - more at https://fonts.google.com */
@import url('https://fonts.googleapis.com/css?family=Open+Sans:400,700');
@import url('https://fonts.googleapis.com/css?family=Merriweather:400,700');
html{
  width: 100%;
  margin: 0;
}
body {
  font-family: "Open Sans", sans-serif;
  font-size: 16px;
  margin: 0;
  background-color: #ffffff;
  height: 100%;
  overflow:auto;
  width: 100%;
}
.header{
  background-image: url("./image/equation.PNG");
  background-position: center;
  background-repeat: no-repeat;
  background-size: cover;
  background-attachment:fixed;
}
#header{
  color: white;
  text-align: center;
  font-size: 25px;
  padding: 90px 0;
}
#menu{
  color: #e7dea2;
  font-size: 17px;
  position: absolute;
  width: 100%;
  list-style: none;
  margin: 0;
  padding: 0;
  text-align: center;
  display: flex;
  justify-content:center;
 top: 310px;
}
.buttonmenu{
  padding-right:1px;
  text-decoration: none;
  color: #464b50;
  background-color: #eee;
  border-radius: 5px;
  font-weight: 600;
  padding: 10px 15px 15px 15px;
  float: left;
  text-align: center;
  margin: 10px 5px
}
.buttonmenu:hover{
  color: #eee;
  transition-duration: 1s;
  background-color: rgba(70, 75, 80, 0.71);
  border-radius: 6px;
}
#agrandir{
  position: fixed;
  right: 3px;
  top: 8px;
  width: 50px;
}
#agrandir:hover{
  width: 100%;
}
.hr{
  width: 10%;
  color: rgb(21, 18, 18)
}