This a small CSS conflict with active theme or other plugins. To solve the issue, please navigate to Dashboard > Forums > Settings > Styles admin page, put the following CSS code in the "Custom CSS Code" textarea, save it, delete all caches and check again. 

.fb-login-button.fb_iframe_widget > span {
   width: 194px !important;
}

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I've implemented facebook login button for my login and registration page.

However , I don't know how to style the button to the center and how to style the facebook button same as Sign in button style.

This is my website:

https://phiforum.com/community/?wpforo=signin