Problem customizing the look and feel

[Ayy Looz]

My problem is that i want to customize my desktop application by changing its look and feel with a customized one that i downloaded from the internet (a .jar file)
But it doesn't work : it compiles and runs fine but i keep on getting the same classic look and feel, here is what i tried in my main() method :

package essailooknfeel;

import javax.swing.JFrame;
import org.jdesktop.application.Application;
import javax.swing.UIManager;
import org.jdesktop.application.SingleFrameApplication;

/**
 * The main class of the application.
 */
public class EssaiLooknFeelApp extends SingleFrameApplication {

    /**
     * At startup create and show the main frame of the application.
     */
    @Override protected void startup() {
        show(new EssaiLooknFeelView(this));
    }

    /**
     * This method is to initialize the specified window by injecting resources.
     * Windows shown in our application come fully initialized from the GUI
     * builder, so this additional configuration is not needed.
     */
    @Override protected void configureWindow(java.awt.Window root) {
    }

    /**
     * A convenient static getter for the application instance.
     * @return the instance of EssaiLooknFeelApp
     */
    public static EssaiLooknFeelApp getApplication() {
        return Application.getInstance(EssaiLooknFeelApp.class);
    }

    /**
     * Main method launching the application.
     */
    public static void main(String[] args) {

        JFrame.setDefaultLookAndFeelDecorated(true);
       
        try{UIManager.setLookAndFeel("com.oyoaha.swing.plaf.oyoaha.OyoahaLookAndFeel");}
        catch (Exception e) {e.printStackTrace(System.out);}



        launch(EssaiLooknFeelApp.class, args);
    }

of course i added the .jar file in my project libraries.
Can anybody tell me what is wrong in my code and how can i get it to work.
Thank you in advance,

Ayy