With a stylesheet sheeted added to SB :-

.root { my-color: #f00;}
.my-label { -fx-background-color: my-color;}

Then a Label with it's styleClass set to "my-label" does not have a red background.

If the stylesheet is changed to: -

.my-label { -fx-background-color: #f00;}

then the label correctly has a red background.