To change the panel's background color, use the following code:
myplot + theme(panel.background = element_rect(fill = 'green', colour = 'red'))
To change the color of the plot (but not the color of the panel), you can do:
myplot + theme(plot.background = element_rect(fill = 'green', colour = 'red'))
See here for more theme details Quick reference sheet for legends, axes and themes.