Define in a stylesheet file the linear gradient rule:
.lineargradient {
-fx-fill: linear (0%,0%) to (0%,100%) stops (0%,derive(-fx-color,-9%)) (100%,derive(-fx-color,-33%));
}

Then, apply it to a Rectangle, from the Look tab.
The Fill property should not allow to change the color fields, and a CSS button should appear.