I propose adding two constants to Application like:

[1/22/13 4:37:05 PM] Jasper Potts: public abstract class Application {
    /**
     * Constant for user agent stylesheet for the "Caspian" theme. Caspian
     * is the theme that shipped as default in JavaFX 2.x.
     */
    public static final String CASPIAN = "CASPIAN";
    /**
     * Constant for user agent stylesheet for the "Modena" theme. Modena
     * is the default theme for JavaFX 8.x.
     */
    public static final String MODENA = "MODENA";

So that in your application you can just say:

public void start(..) {
.....
              setUserAgentStylesheet(MODENA);
.....
....
}