code: def test_theme_gray(self):
      p = self.g + labs(title="Theme Gray") + theme_gray()
  
      assert p == "theme_gray"
  
imports: import os
  import pytest
  from plotnine import aes, element_blank, element_line, element_text, facet_grid, facet_wrap, geom_blank, geom_point, ggplot, labs, theme, theme_538, theme_bw, theme_classic, theme_dark, theme_gray, theme_light, theme_linedraw, theme_matplotlib, theme_minimal, theme_seaborn, theme_tufte, theme_void, theme_xkcd
  from plotnine.data import mtcars
label_desc: Use image node_id 5 for calling the TestThemes obj's underlying member method code with example usage: obj.test_theme_gray() without return types