Have a look at scale_color_manual, the examples should be sufficient. The general structure of tweaking any scale in ggplot2 is to use the appropriate scale function: scale_{aes_name}_{scale_type}, where aes_name can be color, x, or any other aeshetic, and where scale_type can be continuous, discrete, manual, etc.
Googling for ggplot2 legend position led me to this link. Your answer is in there.
Have a look at ?theme, in particular panel.background. The examples there include an example of exactly what you need.
