改变ggtern图中的箭头颜色？

Question

#Load the data
df <- read.table(file = "~/Desktop/PPS-data.txt", header = T)

#Create the plot and store
plot <- ggtern(data = df, aes(x = Xyp, y = XO, z = XY)) + 
               geom_point(aes(fill = Root),
                          size = 4, 
                          shape = 21, 
                          color = "black") + 
               ggtitle("PPS 3-State Model") + 
               labs(fill = "Root States") + 
               theme_tern_rgbw() + 
               theme(legend.position      = c(0,1), 
                     legend.justification = c(0, 1))

#Render
plot

如何更改箭头和轴的颜色，使其与点的颜色匹配？

我研究了ggtern对象的结构，但似乎找不到正确的语法。