diff --git a/src/calibre/gui2/__init__.py b/src/calibre/gui2/__init__.py
index 58fa4219ce..7f75d8d82a 100644
--- a/src/calibre/gui2/__init__.py
+++ b/src/calibre/gui2/__init__.py
@@ -1078,7 +1078,7 @@ class Application(QApplication):
         if iswindows or isosx:
             using_calibre_style = gprefs['ui_style'] != 'system'
         else:
-            using_calibre_style = 'CALIBRE_USE_SYSTEM_THEME' in os.environ
+            using_calibre_style = 'CALIBRE_USE_SYSTEM_THEME' not in os.environ
         if force_calibre_style:
             using_calibre_style = True
         self.using_calibre_style = using_calibre_style