diff --git a/tests/syntax-tests/create_highlighted_versions.py b/tests/syntax-tests/create_highlighted_versions.py
index a9f21ced..5f98d789 100755
--- a/tests/syntax-tests/create_highlighted_versions.py
+++ b/tests/syntax-tests/create_highlighted_versions.py
@@ -8,11 +8,13 @@ import os
 import argparse
 from multiprocessing import Pool
 
+# Avoid 'default' theme because it can choose a different theme based on
+# the appearance settings on macOS.
 BAT_OPTIONS = [
     "--no-config",
     "--style=plain",
     "--color=always",
-    "--theme=default",
+    "--theme=Monokai Extended",
     "--italic-text=always",
 ]