These flags operate at the 'rendering' level, and, supercede the presence of theme elements, therefore,
theme_hidegrid(...) or its aliases will PREVENT rendering of grid elements,
irrespective of whether those grid elements are valid (renderable). From the counter perspective,
theme_showgrid(...) or its aliases will ALLOW rendering of grid elements, subject to those grid
elements being valid (renderable, ie say element_line as opposed to element_blank).
theme_hidegrid or theme_nogrid (alias) is a function which disables both MAJOR and MINOR gridlines.
theme_showgrid_major is a function which enables MAJOR gridlines.
theme_hidegrid_major or theme_nogrid_major (alias) is a function which disables MAJOR gridlines.
theme_showgrid_major is a function which enables MINOR gridlines.
theme_hidegrid_minor or theme_nogrid_minor (alias) is a function which disables MINOR gridlines.
theme_showgrid is a function which enables both MAJOR and MINOR gridlines.