Display node content with frontend theme in admin route
If this is a custom route (defined in a routing.yml file with a custom controller) you are able to do this by defining the route in the routing yaml with
options:
    _admin_route: FALSE

Which will cause the route to be treated as a &quot;frontend&quot; route. However it sounds like this is likely not the case here.
If you simply want to have previewing of standard content, the core preview function (the button next to &quot;Save&quot;) is the standard mechanism to do this.