body {
  background-image: url("images/fond.png");
  background-repeat: repeat;
}

div.cours {
  background-color: rgb(250,250,250);
  background-color: rgba(250,250,250,0.92);
}

div.afaire {
  background-color: rgba(255,255,200,0.75);
}

div.conclusion {
  background-color: rgb(200,250,200);
  background-color: rgba(200,250,200,0.92);
}