changing facet strip colour by grouping variable when using ggh4x::facet_manual

Using ggh4x::facet_grid2 we can colour facet strips by group, see here.

Can the same be done if you are using ggh4x::facet_manual so that for the graph below each strip on the same row (group_var e.g. A, B, C will be blue) will have the same colour?

library(ggplot2)
library(ggh4x)
dat <- data.frame(
  facet = LETTERS[1:8],
  x = 1,
  y = 1,
  group_var = c(1, 1, 1, 
                2, 2,
                3, 3, 3)
)
dat
design = "
ABC
D#E
FGH
"
ggplot(dat, aes(x, y)) +
  geom_point() +
  facet_manual(vars(group_var, facet), design = design,
               strip = strip_split(position = c("left", "top")))

So given the solution to ggh4x::facet_grid2, something like this is needed which doesnt produce what I want:

ggplot(dat, aes(x, y)) +
  geom_point() +
  facet_manual(vars(group_var, facet), design = design, 
               strip = strip_themed(
                 strip_split(position = c("left", "top")),
                 background_x = list(element_rect(fill = "red"),
                                     element_rect(fill = "green"),
                                     element_rect(fill = "blue"))))

thanks

Following ggh4x-strips background_x = list(...) needs to go inside strip_split. ?strip_split -> background_x, background_y
A list() with ?element_rect elements. See the details section in ?strip_themed. background_x then seems to read fill in order of your faceted plots.

library(ggplot2)
library(ggh4x)
dat <- data.frame(
  facet = LETTERS[1:8],
  x = 1,
  y = 1,
  group_var = c(1, 1, 1, 
                2, 2,
                3, 3, 3)
)

design = "
ABC
D#E
FGH
"
# for building the list with "element_rect"
cols <- c("group1" = "dodgerblue",
          "group2" = "red",
          "group3" = "green")

ggplot(dat, aes(x, y)) +
  geom_point() +
  facet_manual(
    vars(group_var, facet),
    design = design,
    strip = strip_split(
      position = c("left", "top"),
      background_x = lapply(cols[dat$group_var], element_rect) # or elem_list_rect(fill = cols[dat$group_var])
    )
  )

giving