figureslibres.cc/user/themes/figureslibres/scss/theme/_header.scss

header{
  position: fixed;
  top: 0;
  left: 0;
  margin: 0 0 0 $marg;
  padding: $marg 0;
  width: calc( #{$Wi} - #{$marg} );
  border-bottom: 1px solid black;
}