You want to change the appearance of facet labels and headers.

With the theming system, set strip.text to control the text appearance and strip.background to control the background appearance (Figure 11-6):

library(gcookbook)  # For the data set

ggplot(cabbage_exp, aes(x=Cultivar, y=Weight)) + geom_bar(stat="identity") +
    facet_grid(. ~ Date) +
    theme(strip.text = element_text(face="bold", size=rel(1.5)),
          strip.background = element_rect(fill="lightblue", colour="black",
                                          size=1))

Figure 11-6. Customized appearance for facet labels

Using rel(1.5) makes the label text 1.5 times the size of the base text size for the theme. Using size=1 for the background makes the outline of the facets 1 mm thick.