diff --git a/theme.py b/theme.py
index f2fc9958..46c760d7 100644
--- a/theme.py
+++ b/theme.py
@@ -122,8 +122,8 @@ def setBackgroundFormat(baseDir: str, name: str, extension: str) -> None:
         with open(cssFilename, 'r') as cssfile:
             css = cssfile.read()
             css.replace('-background.png', '-background.' + extension)
-            with open(cssfilename, 'w+') as cssfile:
-                cssfile.write(css)
+            with open(cssFilename, 'w+') as cssfile2:
+                cssfile2.write(css)
 
 
 def enableGrayscale(baseDir: str) -> None: