If I set -fx-background to a dark color, the text in the Tooltip is white, while the Tooltip's background is yellow, which results in an unreadable Tooltip.
This is due to the ladder in the .root class.

This fixes the problem:

.tooltip .label {
    -fx-text-fill: -fx-dark-text-color;
}

Please integrate it in the caspian.css.

This is related to:
RT-25256