The highlights are caused by CSS styling. Those days have .saturday and .sunday styles assigned to them. Those styles are defined in dateline.css:
.scale-bottom > .saturday, .scale-bottom > .sunday {
-fx-background-color: rgba(0,0,0,0.1) ;
}
.scale-bottom > .saturday:hover, .scale-bottom > .sunday:hover {
-fx-background-color: rgba(0,0,0,0.1) ;
}