Read colors from based definitions to handle theme changes

Fix #186.

I've got a reasonable version now. The downside is that we have to use built-in colors from Fonts and Colors dialog. The list is here http://qscribble.blogspot.dk/2013/10/vs-classification-type-names.html.

The ideal way is to use custom colors and change them appropriately after detecting themes. However, detecting themes requires reading registry and obtaining DTE which are not available at the time `ClassificationFormatDefinition` is initialized.