Answer by Julius Vainora for Rotating x-axis labels and changing theme in ggplot2

That's because of the order: theme_minimal being after theme overrides the latter. Using

ggplot(ToothGrowth, aes(x = dose, y = len)) +  geom_boxplot() + theme_minimal() +  theme(axis.text.x = element_text(angle = 90, hjust = 1))

gives

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