PIVOT-689, add defaultBackgroundColor and defaultForegroundColor in Terra Theme json config file (to be able to set them) with a good fallback value.
Note that even in general Theme there are (general) default values for them.

As a sample, add some (very visible: red, and yellow) values for them.


git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1619713 13f79535-47bb-0310-9956-ffa450edef68