I can't get the docs to build properly so I'll just report an issue instead of a PR...
http://matplotlib.org/users/style_sheets.html

>>> print plt.style.available

should probably be switched to Py3 print()

>>> import numpy as np
>>> import matplotlib.pyplot as plt
>>>
>>> with plt.style.context(('dark_background')):
>>>     plt.plot(np.sin(np.linspace(0, 2*np.pi)), 'r-o')
>>>
>>> # Some plotting code with the default style
>>>
>>> plt.show()

extra parenthesis around 'dark_background', spaces needed in 2 * np.pi, and the >>> prompts with nothing else on the line are not highlighted properly (I would just get rid of all >>> prompts anyways, see e.g. the pyplot tutorial).