This website provides a simple summary of the theme elements that can be set within ggplot2. There should be sufficient information here to change the default settings for graphs within the ggplot2 package.
Interactive and Dynamic Graphics for Data Analysis: With R and GGobi (Use R) Dianne Cook, Deborah F. Swayne
Modern Applied Statistics with S (Statistics and Computing) W.N. Venables, B.D. Ripley