Updates theme-defaults.r (#1555) #1557
                
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addresses issue #1555 where some elements added under certain themes inherit "element_blank" and cannot be updated, e.g.
ggplot(mtcars, aes(wt, mpg)) + geom_point() + theme_classic() + theme(panel.grid.major.y = element_line(colour = "pink", size = 4))update explicitly declares parent element as necessary and children elements as element_blank