a)Preparing look and feel in form of template
b)Modifying the look and feel on the fly.
1)Take a look at following Image, which helps to change the background image of the Master Page, on the fly.
2)Once Composed look is created , Under Site settings -->Look and feel -->Change the Look -->Select recently created look and apply . Here is my site with change of background Image :)
For more insight about .spcolor and .spfont files , see this post and this one