You can add overrides for those styles in any template on your site. They are *not* defined in any CSS file that you have the ability to edit. They are included through the Facebook Javascript library dynamically at run time. So, to fix them, you'll need to use more specific selectors, like:
.jfbclogin .fb_button { ... }
Using a style with .jfbclogin in front should override the defaults provided by Facebook.
Additionally, with the new SCLogin module in v4.2.3, you can use your own custom image or text link if you'd prefer. One option is just taking a picture of the Login With Facebook button how you want it to look and use that
Hope that helps!
Alex