i think you missed line part in below css
.separator *.line {
-fx-border-style: solid;
-fx-border-width: 1px;
}
with boarder width 5px
Tip : use CSS Analayzer to know css class/selector for particular part of node, you can find css analyzer in scenebuilder 1.1 view -> Show css Analyzer
or ctrl+6
shortcut in windows.