How can I customize font on correlogram for publication using ggpairs (GGally)?

Question

I am struggling to customize some aspects of ggpairs correlogram. Here's my code and the correlogram it produces:

lower_plots <- function(data, mapping, ...) {
  ggplot(data = data, mapping = mapping) +
    geom_point(size = 1.5, alpha = 0.5) +
    geom_line(stat='smooth', method = "lm", alpha=0.5, linewidth = 1) 
}

diag_plots <- function(data, mapping, ...) {
  x <<- x + 1
  ggplot(data = data, mapping = mapping) +
    geom_histogram(alpha=0.5,...)
} 

eg<-ggpairs(iris, columns = 2:4, mapping = aes(color = Species, fill=Species),
        upper = list(continuous = wrap(ggally_cor)),
        diag = list(continuous = wrap(diag_plots, bins = 15)),
        lower = list(continuous = wrap(lower_plots, se=FALSE)))

for(i in 1:eg$nrow) {
  for(j in 1:eg$ncol){
    eg[i,j] <- eg[i,j] + 
      scale_fill_manual(values=c("#F8766D", "#00BA38", "#619CFF")) +
      scale_color_manual(values=c("#F8766D", "#00BA38", "#619CFF"))
  }
}

eg<-eg + theme(panel.background = element_blank(),
                   strip.background = element_rect(fill = "white"),
                   strip.text = element_text(hjust = -1, margin=margin(l=0)),
                   panel.grid.major = element_blank(), 
                   panel.border = element_rect(linetype = "solid", colour = "black", fill = NA))
eg

Plot from supplied code using Iris data set

Here's the plot I am trying to produce. I am trying to customize the Pearson correlation coefficient labels to italicize for species names. I also have complicated units (greek, sub/superscript, and italics) in some of the axis labels, which I have been unable to accommodate.

Correlogram I am trying to create