I'm trying to create a black and white ScrollPane in JavaFX. I already created a CSS file, which works pretty fine. Except for this little square:

Whatever I try, I'm not able to turn it black. Here is my CSS file:

.scroll-pane {
    -fx-background-color: black;
}

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

.increment-button, .decrement-button {
    -fx-background-color: transparent;
    -fx-border-color: transparent;
}

.scroll-bar:horizontal .track,
.scroll-bar:vertical .track{
    -fx-background-color: transparent;
    -fx-border-color: transparent;
    -fx-background-radius: 0em;
}

.scroll-bar:horizontal .thumb,
.scroll-bar:vertical .thumb {
    -fx-background-color: white;
    -fx-background-radius: 5em;
}

Is there a way to change the color of this square?