Color::Theme::Role is for class that wants to support color themes. Color theme
is represented as a structure according to the specification described in
Color::Theme.

Color themes are put in modules under Color::Theme::Themes:: (configurable using
color_theme_module_prefix attribute). Each color theme modules can contain one
or more color themes. The module must define a package global variable named
%color_themes that contain color themes keyed by their names.
