轴刻度标签（R，ggplot2）内的不同字体样式和颜色

Question

我正在使用以下代码。

 library(ggplot2)
 library(dplyr)
 mtcars$carb <- as.factor(mtcars$carb)
 mtcars$carb <- as.factor(mtcars$carb)
 mtcars$am <- as.factor(mtcars$am)
 additions <- data.frame("carb" = c(1,2,3,4,6,8), 
      "words_eng" = c("one","two","three","four","six","eight"), 
      "words_spa" = c("uno","dos","tres","cuatro","seis","ocho"))
 mtcars2 <- merge(mtcars, additions, by = "carb")
 mtcars2 <- select(mtcars2, "carb", "am", "mpg", "words_eng", "words_spa")
 mtcars2 <- mutate(mtcars2, joined = paste0(words_eng, "_", words_spa, "_", carb))
 ggplot(mtcars2) + 
    geom_boxplot(aes(x = joined, y = mpg, fill = am), 
                 position = position_dodge(0.9)) +
 guides(fill = guide_legend(direction = "horizontal")) + 
 theme(axis.text.x = element_text(angle=90, vjust=0.5, hjust=1))

结果是：

如何修改x轴刻度标签内的字体样式和颜色，以使“一个”，“两个”，“三个”等以蓝色粗体显示，而“ uno”，“ dos”， tres”将以红色粗体显示，并且“ 1”，“ 2”，“ 3”等保留原样吗？谢谢。