Depends on the VERSION of Showcase. I JUST (in SC 2.3.0 RC 1) introduced the ability to set a specific style for Showcase. Individual Categories have been able to set a specific style for that category only for several versions now.
Anyway, if you are running the NEWER version of showcase, then there is THIS setting on the Showcase General Options Tab. If you set a style here, ALL showcase pages will be rendered in that particular style no matter what the viewing users style choice is set to. The only exception to this rule is when viewing a CATEGORY page that has a specific style set for that category only. Early versions of showcase did not have this setting and will render with the style that the viewing user has set.
As mentioned above, CATEGORY's have the ability to also set a specific style that will be forced on the viewing user. You might want to double check your categories to ensure none of them are set to Override and display the Default Style (this is a common mistake)