Unfortunately, you can't do that. Facebook used to have a lot more abilities to add custom CSS files to most of their widgets. However, over the last 6 months or so, they've been slowly removing that ability from their widgets because some administrators abused the ability to overwrite the styles of them. Abuse included changing the output so drastically that the button did not 'look' like a Like button, for instance, which could have the effect of tricking a user to clicking it, thinking it had a different functionality.
Right now, the only options are for the Light and Dark colorschemes. We agree, this isn't a great choice, and Facebook has heard a lot of feedback to add back in some options to customize. We'll definitely update our extensions if those options become available.
Sorry for the bad news. We wish it was easier to customize the output as well.