If, using CSS, I specify the fill rule for one axis, the other axis is automatically filled with the black color.

ie: the following rule makes the vertical axis filled with red.

.chart .axis:left {
    -fx-background-color: red;
}

Then the horizontal axis is filled with black.

Resp, if instead I specified the following rule which makes the horizontal axis filled green

.chart .axis:bottom {
    -fx-background-color: green;
}

Then the vertical axis is filled black.

See attached screenshot for some example.