You should read the Skinning JavaFX Applications with CSS tutorial and the JavaFX CSS Reference Guide.
Excerpt:
You can create a class style by adding a definition for it to your style sheet. Example 5 defines a new style in controlStyle1.css called .button1.
Example 5 Define a New Style
.button1{
-fx-text-fill: #006464;
-fx-background-color: #DFB951;
-fx-border-radius: 20;
-fx-background-radius: 20;
-fx-padding: 5;
}
To assign this class style to a node, use the getStyleClass().add() sequence of methods. Example 6 shows the .button1 style assigned to the Accept button.
Button buttonAccept = new Button("Accept");
buttonAccept.getStyleClass().add("button1");