With the new theme, I moved away from the "Prev/Next" navigation in the details view, and kept navigation in the list view. It would be fairly easy to add this sort of navigation back, as it is defined in the Dataface_DetailsController.html template:
You could inject this using a block or slot. Or you may notice that Xataface templates support the special smarty tags that can be added to any template:
{prev_link}
{next_link}
{jump_menu}
Which will do what the names imply.
You might need to tweak CSS styles a bit to make them fit the look.
The root details template is this one:
Another good way to reintroduce this would be behind a config option or preference to include the details controller somewhere (or just prev/next links).
If you end up going this route, please make a PR and I'll merge it into the core.