I added the following stylesheet to hightlight pies of PieChart under mouse cursor:

.chart-pie:hover {
    -fx-border-color: WHITE;
    -fx-border-width: 2;
}

There are two problems:
1. Some of pies are drawn behind the others so that their white borders are not fully visible
2. Topmost pie has sharp stroke join in the center of the pie so that it doesn't look pretty. I believe round join is more suitable here.