Hi, David. Are you referring to the “Events template” setting (https://theeventscalendar.com/knowledgebase/wp-admin-settings-overview/#display)?
If yes, this can affect the layout but won’t affect the CSS.
The “Default stylesheet used for events templates” setting can affect the CSS that loads on the page but it’s not an “either-or” thing between the theme and our plugins.
Does this information help clarify things for you?
If not, would you be able to share an annotated screenshot, screen capture video, and/or relevant URLs to clarify exactly what you’re experiencing?