You can add + theme_classic() to your ggplot to see what it does. ggplot has several built-in "themes" that customize the overall look of the plot in various ways. The default theme is theme_grey(). You can change this for all future plots by running, for example:
theme_set(theme_classic())
To see available themes in ggplot, see the help for any theme. For example, run ?theme_classic(). The help page that opens will list all of the available themes. The ggthemes package has more, and you can of course create your own.
To tweak any theme, add a theme() statement to a ggplot after setting the overall theme. For example:
ggplot(mtcars, aes(mpg, hp)) +
geom_point() +
theme_classic() +
theme(axis.text.x=element_text(size=rel(1.1)),
panel.border=element_rect(fill=NA))
See here and here for web versions of theme help files.
Here are some examples of how ggplots look with various themes.