Will the app cause any changes to my store theme?

No, Video Background app not needs to modify any of your templates to run nicely in your store.

We use a Shopify programming feature called ScriptTag (see https://help.shopify.com/en/api/reference/online-store/scripttag for details) that allows our app to be injected dynamically by Shopify when a page loads.

While the ScriptTag feature is very convenient, scripts that run that way have to wait till other content on the page is loaded, what can be perceived as the app being slow.