Java SE (Java Platform, Standard Edition)

I customized the style of the scrollbar following mainly this article.

However on the increment/decrement button I get a border when hovering over it. How can I remove this effect?

.scroll-bar:vertical {
    -fx-background-color:transparent;
}

.scroll-bar:vertical .decrement-arrow {
    -fx-background-repeat: no-repeat;
    -fx-background-image : url('scrollUp.png');
    -fx-shape: "";
    -fx-padding: 16px, 15px;
    -fx-background-color:transparent;
}
.scroll-bar:vertical .increment-arrow {
    -fx-background-repeat: no-repeat;
    -fx-background-image : url('scrollDown.png');
    -fx-shape: "";
    -fx-padding: 16px, 15px;
    -fx-background-color:transparent;
}
.scroll-bar:vertical .increment-arrow:hover,
.scroll-bar:vertical .decrement-arrow:hover {
    -fx-border-style: none;
}

.scroll-bar:horizontal .thumb,
.scroll-bar:vertical .thumb {
    -fx-background-color:derive(rgb(234, 193, 23),90%);
    -fx-background-insets: 0, 0, 0;
    -fx-background-radius: 0em;
}

Having a style definition for hover has no effect. What I have seen with ScenicView is that on the Region underlying the button a DropShadow effect is defined. Might that be the cause?