library(tidyverse)
df <- data.frame(x = c(1,2),
           y = c(1,2))
df %>% ggplot(aes(x, y)) +
  geom_area() + theme(
    panel.grid = element_line(color = "red")
  )

grd_x <- seq(1, 2, length.out = 9)
grd_y <- seq(0, 2, length.out = 9)

df %>% ggplot(aes(x, y)) +
  geom_area() +
  geom_hline(yintercept = grd_y, col = "red") +
  geom_vline(xintercept = grd_x, col = "red")

library(tidyverse)
df <- data.frame(x = c(1,2),
                 y = c(1,2))
df %>% ggplot(aes(x, y)) +
  geom_area() +
  theme(panel.grid = element_line(color = "red"),
    panel.ontop = TRUE, panel.background = element_rect(color = NA, fill = NA)
  )