How to change color of facet borders when using facet_grid

Although a year late, I found this to be an easy fix:

ggplot(mpg, aes(cty, hwy, color = factor(year)))+   geom_point()+  facet_grid(cyl ~ drv) +  theme(panel.margin=unit(.05, "lines"),        panel.border = element_rect(color = "black", fill = NA, size = 1),         strip.background = element_rect(color = "black", size = 1))

UPDATE 2021-06-01

As of ggplot2 3.3.3, the property panel.margin is deprecated, and we should use panel.spacing instead. Therefore, the code should be:

ggplot(mpg, aes(cty, hwy, color = factor(year)))+   geom_point()+  facet_grid(cyl ~ drv) +  theme(panel.spacing = unit(.05, "lines"),        panel.border = element_rect(color = "black", fill = NA, size = 1),         strip.background = element_rect(color = "black", size = 1))

You might need to use ggplot's layout table and gtable functions.

library(ggplot2)library(gtable)library(grid)p <- ggplot(mtcars, aes(mpg, wt)) + geom_point() +      facet_grid(am ~ cyl)## Get the plot grobgt <- ggplotGrob(p)## Check the layoutgtable_show_layout(gt)   # Vertical gaps are in columns 5 and 7                         # and span rows 4 to 9                         # Horizontal gap is in row 8                         # and spans columns 4 to 9## To automate the selection of the relevant rows and columns:## Find the panels in the layout## (t, l, b, r refer to top, left, bottom, right);## The gaps' indices are one to the right of the panels' r index (except the right most panel);## and one below the panels' b index (except the bottom most panel);## Rmin and Rmax give the span of the horizontal gap;## Bmin and Bmax give the span of the vertical gap;## cols and rows are the columns and row indices of the gaps.panels = subset(gt$layout, grepl("panel", gt$layout$name), t:r)# The span of the horizontal gapRmin = min(panels$r)Rmax = max(panels$r) + 1#The span of the vertical gapBmin = min(panels$t) - 1Bmax = max(panels$t)# The columns and rows of the gapscols = unique(panels$r)[-length(unique(panels$r))] + 1rows = unique(panels$t)[-length(unique(panels$t))] + 1# The grob - orange rectangleg = rectGrob(gp = gpar(col = NA, fill = "orange"))## Add orange rectangles into the vertical and horizontal gapsgt <- gtable_add_grob(gt,       rep(list(g), length(cols)),      t=Bmin, l=cols, b=Bmax)gt <- gtable_add_grob(gt,       rep(list(g), length(rows)),      t=rows, l=Rmin, r=Rmax)## Draw itgrid.newpage()grid.draw(gt)

Try this--you can control it with strip.background, formated with an element_rect call.

qplot(hwy, cty, data = mpg) +  facet_grid(. ~ manufacturer) +     theme(strip.text.x = element_text(size = 8,                                    colour = "red",                                     angle = 90),        strip.background = element_rect(fill = "darkblue",                                        colour = NA)  )