As an end user: I chose my web browser. I know how things tend to look in it. I know that this is not the same as how they look by default in other browsers. I do not care. I actively want it the widgets to look familiar, because that means I can readily parse the UI, and don't have to think about your design.
I actively do not want you to even try to "make it look the same everywhere", because the way you want it to look will in general be a way that degrades functionality for me. When you try, you send the message that you know better than me about how I want the GUI on my computer to work; and that makes me less inclined to use your site, and thus your product.