input: Write a python function to get the user's preferred background color.
  assert get_background_color(user_preferences) == '#FFFFFF'
  assert get_background_color({'language': 'english', 'font_size': 12, 'background_color': '#000000'}) == '#000000'
  assert get_background_color({'language': 'english', 'font_size': 12, 'background_color': '#FF5733'}) == '#FF5733'
output: def get_background_color(user_preferences):
      return user_preferences.get('background_color', '#FFFFFF')