PIVOT-689, refactoring assignment of default background/foreground color depending on the theme is dark (or not).
Note that common methods currently are all in ComponentSkin, but no additional methods have been put in Theme (but they could be).



git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1619155 13f79535-47bb-0310-9956-ffa450edef68