Simplify theme structure

[AmaranthineCodices]

Right now, themes have a bunch of really specific properties:

ButtonHoverColor = Colors.Lighten("Blue500", 1);
ButtonPressColor = Colors.Darken("Blue500", 1);
FlatButtonHoverColor = Colors.Lighten("Blue500", 4);
FlatButtonPressColor = Colors.Lighten("Blue500", 3);
FlatButtonColor = Colors.White;
ButtonColor = Colors.Blue500;
-- ...

These can probably be ripped out and replaced with various Colors.Lighten and Colors.Darken calls based on the theme primary/accent colors.