diff --git a/news/199.bugfix b/news/199.bugfix
new file mode 100644
index 00000000..052c0886
--- /dev/null
+++ b/news/199.bugfix
@@ -0,0 +1,2 @@
+Fix unclosed file when reading manifest.cfg
+[petschki]
diff --git a/src/plone/app/theming/utils.py b/src/plone/app/theming/utils.py
index f3ca2231..e494e242 100644
--- a/src/plone/app/theming/utils.py
+++ b/src/plone/app/theming/utils.py
@@ -471,9 +471,8 @@ def getThemeFromResourceDirectory(resourceDirectory):
     """
     name = resourceDirectory.__name__
     if resourceDirectory.isFile(MANIFEST_FILENAME):
-        manifest = getManifest(
-            resourceDirectory.openFile(MANIFEST_FILENAME), MANIFEST_FORMAT
-        )
+        with resourceDirectory.openFile(MANIFEST_FILENAME) as manifest_fp:
+            manifest = getManifest(manifest_fp, MANIFEST_FORMAT)
     else:
         manifest = {}