Will this app cause any changes to my store theme or styles?
The app 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.
If your theme has another built-it shipping calculator it may be disabled automatically when Shipping Rates Calculator+ is enabled. The theme calculator can be enabled again easily from the theme preferences.
Every other configuration is done from our app code. You can tune the styles of the app by using the app preferences page.