Allow form.widgets.RenderTemplateWidget to use the theme object

Author: cjmayo

flask_admin/base.py

@@ -598,6 +598,8 @@ def __init__(
         # Register with application
         if app is not None:
             self._init_extension()
+            # Make theme available via current_app
+            app.config['theme'] = self.theme
 
     def _validate_admin_host_and_subdomain(self):
         if self.subdomain is not None and self.host is not None:

flask_admin/form/widgets.py

@@ -108,6 +108,7 @@ def __call__(self, field, **kwargs):
                 "_gettext": gettext,
                 "_ngettext": ngettext,
                 "h": h,
+                "theme": current_app.config["theme"],
             }
         )