Is possible to customize progress bar?

Well, i see some questions like mine in here, but i don't really understand if is possible to customize the Progress Bar Behavior.

I found the option "Set Css Style" but i realy don't know how to use.

I understand that her appearance is determined by the browser, but if I want a proper fixed style using this behavior, what should I do?

Ashley can you explain a little bit more about this behavior?

The progress bar is deliberately styled to look like a native control. If you want something different, it's easy to make your own with a tiled background.