Change facet label text and background colour

You can do:

ggplot(A) +
  geom_point(aes(x = x, y = y)) +
  facet_wrap(~z) +
  theme_bw()+
  theme(strip.background =element_rect(fill="red"))+
  theme(strip.text = element_text(colour = 'white'))

For anyone else looking to change individual facet labels, there's a solution here:

g <- ggplot_gtable(ggplot_build(p))
stripr <- which(grepl('strip-r', g$layout$name))
fills <- c("red","green","blue","yellow")
k <- 1
for (i in stripr) {
  j <- which(grepl('rect', g$grobs[[i]]$grobs[[1]]$childrenOrder))
  g$grobs[[i]]$grobs[[1]]$children[[j]]$gp$fill <- fills[k]
  k <- k+1
}
grid::grid.draw(g)