Accessed in Appearance → Customize → Page Settings → Password protected page template .
In theme there is defined Password protected page template that is called when passworded post is requested.
Theme have default styling for such situation, and you can change background image for this page.
In Rife Pro you have another option to create custom page and use it as your Password protected page by selecting it in these settings.
Password form on such custom page will be added at end of this page content.