You want to change the appearance of theme elements.

To modify a theme, add theme() with a corresponding element_xx object. These include element_line, element_rect, and element_text. The following code shows how to modify many of the commonly used theme properties (Figure 9-6):

library(gcookbook) # For the data set

# Base plot
p <- ggplot(heightweight, aes(x=ageYear, y=heightIn, colour=sex)) + geom_point()

# Options for the plotting area
p + theme(
    panel.grid.major = element_line(colour="red"),
    panel.grid.minor = element_line(colour="red", linetype="dashed", size=0.2),
    panel.background = element_rect(fill="lightblue"),
    panel.border = element_rect(colour="blue", fill=NA, size=2))

# Options for text items
p + ggtitle("Plot title here") +
    theme(
    axis.title.x = element_text(colour="red", size=14),
    axis.text.x  = element_text(colour="blue"),
    axis.title.y = element_text(colour="red", size=14, angle = 90),
    axis.text.y  = element_text(colour="blue"),
    plot.title = element_text(colour="red", size=20, face="bold"))

# Options for the legend
p + theme(
    legend.background = element_rect(fill="grey85", colour="red", size=1),
    legend.title = element_text(colour="blue", face="bold", size=14),
    legend.text = element_text(colour="red"),
    legend.key = element_rect(colour="blue", size=0.25))

# Options for facets
p + facet_grid(sex ~ .) + theme(
    strip.background = element_rect(fill="pink"),
    strip.text.y = element_text(size=14, angle=-90, face="bold"))
    # strip.text.x is the same, but for horizontal facets