diff --git a/invenio_theme/ext.py b/invenio_theme/ext.py
index 7ee6896..a1f0407 100644
--- a/invenio_theme/ext.py
+++ b/invenio_theme/ext.py
@@ -3,6 +3,7 @@
 # This file is part of Invenio.
 # Copyright (C) 2015-2018 CERN.
 # Copyright (C) 2022-2023 Graz University of Technology.
+# Copyright (C) 2025 Northwestern University.
 #
 # Invenio is free software; you can redistribute it and/or modify it
 # under the terms of the MIT License; see LICENSE file for more details.
@@ -14,6 +15,13 @@
 
 from . import config
 from .icons import ThemeIcons
+from .views import (
+    insufficient_permissions,
+    internal_error,
+    page_not_found,
+    too_many_requests,
+    unauthorized,
+)
 
 
 class InvenioTheme(object):
@@ -76,3 +84,44 @@ def _generator_func_or_str():
     def icons(self):
         """Return icons."""
         return ThemeIcons(self.app.config["APP_THEME"], self.app.config["THEME_ICONS"])
+
+
+class InvenioApiTheme:
+    """Invenio theme extension for API app."""
+
+    def __init__(self, app=None):
+        r"""Extension initialization.
+
+        :param app: An instance of :class:`~flask.Flask`.
+        """
+        if app:
+            self.init_config(app)
+
+    def init_config(self, app):
+        """Initialize configuration.
+
+        :param app: An instance of :class:`~flask.Flask`.
+        """
+        _vars = ["BASE_TEMPLATE", "COVER_TEMPLATE", "SETTINGS_TEMPLATE"]
+
+        for k in dir(config):
+            if k.startswith("THEME_") or k in _vars:
+                app.config.setdefault(k, getattr(config, k))
+
+        # Set THEME_<name>_TEMPLATE from <name>_TEMPLATE variables if not
+        # already set.
+        for varname in _vars:
+            theme_varname = f"THEME_{varname}"
+            if app.config[theme_varname] is None:
+                app.config[theme_varname] = app.config[varname]
+
+        app.config.setdefault("ADMIN_BASE_TEMPLATE", config.ADMIN_BASE_TEMPLATE)
+
+
+def finalize_app(app):
+    """Finalize app."""
+    app.register_error_handler(401, unauthorized)
+    app.register_error_handler(403, insufficient_permissions)
+    app.register_error_handler(404, page_not_found)
+    app.register_error_handler(429, too_many_requests)
+    app.register_error_handler(500, internal_error)
diff --git a/invenio_theme/views.py b/invenio_theme/views.py
index ee7f64b..a75d74d 100644
--- a/invenio_theme/views.py
+++ b/invenio_theme/views.py
@@ -3,6 +3,7 @@
 # This file is part of Invenio.
 # Copyright (C) 2015-2018 CERN.
 # Copyright (C) 2022-2023 Graz University of Technology.
+# Copyright (C) 2025 Northwestern University.
 #
 # Invenio is free software; you can redistribute it and/or modify it
 # under the terms of the MIT License; see LICENSE file for more details.
@@ -24,12 +25,6 @@ def create_blueprint(app):
     if app.config["THEME_FRONTPAGE"]:
         blueprint.add_url_rule("/", "index", view_func=index)
 
-    app.register_error_handler(401, unauthorized)
-    app.register_error_handler(403, insufficient_permissions)
-    app.register_error_handler(404, page_not_found)
-    app.register_error_handler(429, too_many_requests)
-    app.register_error_handler(500, internal_error)
-
     return blueprint
 
 
diff --git a/run-tests.sh b/run-tests.sh
index 5702ade..06d8603 100755
--- a/run-tests.sh
+++ b/run-tests.sh
@@ -1,3 +1,4 @@
+#!/usr/bin/env sh
 # -*- coding: utf-8 -*-
 #
 # This file is part of Invenio.
diff --git a/setup.cfg b/setup.cfg
index f95e37a..17456d7 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -3,6 +3,7 @@
 # This file is part of Invenio.
 # Copyright (C) 2015-2024 CERN.
 # Copyright (C) 2022-2024 Graz University of Technology.
+# Copyright (C) 2025 Northwestern University.
 #
 # Invenio is free software; you can redistribute it and/or modify it
 # under the terms of the MIT License; see LICENSE file for more details.
@@ -46,8 +47,12 @@ invenio_assets.webpack =
     invenio_theme = invenio_theme.webpack:theme
 invenio_base.apps =
     invenio_theme = invenio_theme:InvenioTheme
+invenio_base.api_apps =
+    invenio_theme = invenio_theme:InvenioApiTheme
 invenio_base.blueprints =
     invenio_theme = invenio_theme.views:create_blueprint
+invenio_base.finalize_app =
+    invenio_theme = invenio_theme.ext:finalize_app
 invenio_i18n.translations =
     messages = invenio_theme
 
diff --git a/tests/conftest.py b/tests/conftest.py
index 683de50..56abb60 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -27,6 +27,7 @@
 from invenio_i18n.views import create_blueprint_from_app
 
 from invenio_theme import InvenioTheme
+from invenio_theme.ext import finalize_app
 from invenio_theme.views import create_blueprint
 
 
@@ -114,6 +115,7 @@ def tear_down():
     InvenioTheme(app)
 
     app.register_blueprint(create_blueprint(app))
+    finalize_app(app)
     return app