Failed to change the subplot background when generating a grouped correlation matrix heat map using ggplot2 facet

Question

I am a newbie for stack Overflow and r language.

Here is my problem. I now have a dataframe with one variable called Type and other 14 variables whose correlation matrix heatmap needed to be calculated.

origin dataset

I already have an overall format using ggplot2, and the theme is default theme_grey but fine for me to view. The code is :

m<- melt(get_lower_tri(round(cor(xrf[3:16], method = 'pearson', use = 'pairwise.complete.obs'), 2)),na.rm = TRUE)
ggplot(m, aes(Var1, Var2, fill = value)) + 
  geom_tile() + 
  scale_fill_gradient2(low = 'skyblue4', 
                       high = 'coral2', 
                       mid = 'white', 
                       midpoint = 0,
                       limit = c(-1, 1), 
                       space = "Lab", 
                       name = 'Person\nCorrelation') + 
  theme_grey()+
  coord_fixed() +
  theme(axis.title = element_blank())

The result is fine and the background looks good to view.

But when I managed to generate a grouped correlation matrix heatmap, I found that no matter how hard I tried (using theme(panel.background = element_rect()) or theme(panel.background = element_blank())), the subplot backgrounds won’t change and remain this ugly grey which is even different from the overall one.

Here is my code:

Type = rep(c('(a)', '(b)', '(c)','(d)', '(e)', '(f)', '(g)', '(h)', '(i)', '(j)'), each = 14^2)

# Get lower triangle of the correlation matrix
get_lower_tri<-function(x){
  x[upper.tri(x)] <- NA
  return(x)
}

df2 <- do.call(rbind, lapply(split(xrf, xrf$Type), 
                             function(x) melt(get_lower_tri(round(cor(x[3:16], method = 'pearson', use = 'pairwise.complete.obs'), 2)),na.rm = FALSE)))

my_cors <- cbind(Type,df2)

my_cors %>% 
  ggplot(aes(Var1, Var2, fill = value)) + 
  geom_tile() + 
  scale_fill_gradient2(low = 'skyblue4', 
                       high = 'coral2', 
                       mid = 'white', 
                       midpoint = 0,
                       limit = c(-1, 1), 
                       space = "Lab", 
                       name = 'Person\nCorrelation') + 
  theme_grey()+
  coord_fixed() +
  theme(axis.title = element_blank(),
        panel.background = element_rect(fill = 'grey90',colour = NA))+
  facet_wrap("Type",ncol = 5, nrow = 2)

Isn’t the facet subplot backgrounds the same as the overall one if using the same theme? And how can I change it?

Update:sorry! It’s my first time to raise a question and it’s not a good one!

xrf is my original dataset...But now I have figured out why thanks to Tjebo and those who comment my faulty questions.It’s very instructive to me!!

scale_fill_gredient2(...,na.value = 'transparent') will solve it.The default value of this parameter is "grey50" which I took as the background color.

I am truly sorry for asking such a silly question, and I really really appreciate you guys’s nice comment for a rookie! Thank you guys!