Set the theme background color
This overrides part of the previous commit. The color of the background
is now set in the theme rather than having the theme background be null
and then set the background for each activity/layout individually.
The new background color is very similar to default Holo light color.
For some reason, the default Holo light background fades from gray to
white when scrolling down a list, which is bad since it will blend with
the card background.