How to use custom html button for fblogin?

Hello,
i'd like to know if there's a way to use a custom html button instead of this one (attachment)

to use in our website.
As you can see the results are really bad with an image so we would like to use our theme UI to manage buttons.

Are there any possible tricks? thank you.

Hi Melisssa, i've already read the documentation but my question was different,
i don't want to use images as button because i need to render an html button with its own css.

so the question is how could i make a workaround to use html buttons. I know how to create my own html button but i'd need to know what is the file that generates the button in jfb.

thank you, it is working great!