diff --git a/src/calibre/gui2/__init__.py b/src/calibre/gui2/__init__.py
index a67dc74680..151f5b59ab 100644
--- a/src/calibre/gui2/__init__.py
+++ b/src/calibre/gui2/__init__.py
@@ -1292,8 +1292,8 @@ class Application(QApplication):
         return ans
 
     def stylesheet_for_line_edit(self, is_error=False):
-        return 'QLineEdit { border: 2px solid %s; border-radius: 3px }' % (
-            '#FF2400' if is_error else '#50c878')
+        col = '#FF2400' if is_error else '#50c878'
+        return f'QLineEdit {{ border: 2px solid {col}; border-radius: 3px }}'
 
     def _send_file_open_events(self):
         with self._file_open_lock: