I have created a new theme in /usr/share/plymouth/themes and added a .png image to it. Hence, while shutting down and booting up it displays that image. Now I want to add a progress bar as well. How to add it into the theme??
Thanks

What do you mean? A 0% to 100% progress bar? If so, what progress would that be showing? I'm not sure that it can show anything unless you actually have a program or script running which can relate that progress to something which can then illustrate a graphical progress.

I want to add the bootup progress bar.. When the System boots up the tri-colour progress bar it shows by default, so Similar kind of a progress bar I want to add to my new theme. How do I add it?

Well you can add new PNG files to the theme directory, but you'll also have to modify the program using the theme so as to cause it to display the new image.

I am not getting what you said.. Can you please explain in detail.

Actually I already did.

In the themes directory you created a bunch of replacement PNG files for the existing ones, correct? close.png, close_unfocused.png, close_unfocused_pressed.png, maximize.png, .... and so forth?
To create a progress bar that would be used, there would already have to be existing progress bar files, or instead you could create some new files; however if you create new files for some imagery or animation to be used by the window manager, you'd then also have to modify the window manager so that it would make use of these new image files.

It is also possible that the progress bar you're referring too may not be part of your desktop window manager, and that it may be part of the bootloader.