If I apply this style setting
-fx-font: 30 'Arial Black';
to a label, for example, then the size is set correctly but the font familiy is silently ignored.
If I however write it like this
-fx-font: 30 "Arial Black";
both, the size and the font family, are set correctly.

This raises two questions:
Why is the string in single quotes not accepted and why is there no error indication?
The CSS documentation clearly sais:
"Strings can either be written with double quotes or with single quotes."