Please note: to perform the following action you must have already entered in at least one citation with SmartCite. Without any citations in place, you will not be able to edit your style.
To edit or add a new citation style in SmartCite, head over to the "Bibliography" pane. Once there, you'll see an option to "use custom style"
This will then allow you to add in a style which is not currently available in SmartCite or create a custom style via Citation Style!