Flextable color background cells based on a variable in multiple line header?

[SandraG]

I am trying to make a conditional background coloring with the flextable package.
First I created this table that contains a 2 line header

Cancer <- read.table("http://users.stat.ufl.edu/~aa/cat/data/Cancer.dat",
                     header = TRUE, stringsAsFactors = TRUE)
library(dplyr)
library(tidyr)
library(scales)
library(flextable)

cancerCountWide <- Cancer %>% 
  select(-risktime) %>% 
  pivot_wider(id = time, names_from = c(histology, stage), 
              values_from=count) %>%
  mutate(`histo` = " ") %>% 
  select(time, histo, `1_1`, `2_1`, `3_1`, everything())


my_header <- data.frame(
  col_keys = c("time", "histo", "blank1", "1_1", "2_1", "3_1", "blank2", "1_2", "2_2", "3_2", "blank3", "1_3", "2_3","3_3"),
  line2 = c("Follow-up", "Histology", "", rep("I", 3), "", rep("II", 3), "", rep("III", 3)),
  line3 = c("Follow-up", "Disease Stage", rep(c("", "1", "2", "3"), 3))
)

flextable(cancerCountWide, col_keys = my_header$col_keys) %>% 
  set_header_df(
    mapping = my_header,
    key = "col_keys"
  )%>%
  theme_booktabs() %>% 
  merge_v(part = "header") %>% 
  merge_h(part = "header") %>% 
  align(align = "center", part = "all") 

With it I obtained the next output:

I would like to color the cell backgrounds based on the first line of the header. Let's say I would like to color orange the diseases stages (1 , 2 and 3) for the histology group II that have values higher than 5. I thus added the bg option to the previous code:

flextable(cancerCountWide, col_keys = my_header$col_keys) %>% 
  set_header_df(
    mapping = my_header,
    key = "col_keys"
  )%>%
  theme_booktabs() %>% 
  merge_v(part = "header") %>% 
  merge_h(part = "header") %>% 
  align(align = "center", part = "all") 
%>% bg(i = ~ II >5, j= 'II', bg = "orange", part="body")

but I get the next error:

** Error in eval(as.call(f[[2]]), envir=data) : object 'II' not found**

It seems that since the header is split into different lines the variables cannot be selected. How can I fix this?

Thank you

S