Will this app cause any changes to my store theme or styles?

Frequently Bought Together has a minimal impact on the theme templates as it only requires a target element for placing the widget. The app can also be configured to be tied to an existing element on your page, leaving your product template untouched.

In addition, the app doesn't change the theme stylesheet to configure the widget because everything is done from our app code. You can tune the styles of the app by using the app preferences page.