If Styler.border_style is set to 'default_grid', it throws an error if Styler.bg_color is not None (styler.py, line 83). However, before that there seems to be a conversion done to bg_color that turns None values into white (styler.py, line 73), making it such that the option 'default_grid' will always throw an error, which is not consistent with the error message presented ("bg_color and border_type=default_grid can not be used together").
If Styler.border_style is set to 'default_grid', it throws an error if Styler.bg_color is not None (styler.py, line 83). However, before that there seems to be a conversion done to bg_color that turns None values into white (styler.py, line 73), making it such that the option 'default_grid' will always throw an error, which is not consistent with the error message presented ("bg_color and border_type=default_grid can not be used together").