fixed background of code pieces (#1715)

docs/conf.py

@@ -95,7 +95,7 @@ language = None
 exclude_patterns = ['_build', 'Thumbs.db', '.DS_Store']
 
 # The name of the Pygments (syntax highlighting) style to use.
-pygments_style = 'sphinx'
+pygments_style = 'default'
 
 # If true, `todo` and `todoList` produce output, else they produce nothing.
 todo_include_todos = False