gistfile1.js

var Theme = {
    'art-widget': {
        styles: {
            normal: { roundness: 4, backgroundOpacity: .7 },
            focused: { backgroundOpacity: .9 }
        },
        'art-button': { // should inherit 'art-widget' styles
            normal: { roundness: 2, backgroundOpacity: 1 }
        },
        'art-input': { // should inherit 'art-widget' styles
            normal: { roundness: 4, backgroundOpacity: 1 }
        }
    }
};

var ThemeManager = new new Class({

    initialize: function() {
        this.widgets = [];
    },

    register: function(instance) {
        this.widgets.include(instance);
    },

    changeTheme: function() {
        this.widgets.each(function(widget) {
            this.draw(widget);
        }, this);
    },

    draw: function(widget, state) {
        var el = $(widget);
        // widget.className should by splitted and each style should by mixed and down cascaded
        var theme = Theme[widget.className][state || 'normal'];
        // applay canvas or what-ever styles
    }

});

var Widget = new Class({

    Implements: [Options, Events],

    className: 'art-widget',

    options: {
        theme: Theme
    },

    initialize: function(options) {
        this.setOptions(options);
        this.element = new Element('div');
        ThemeManager.register(this);
    },

    render: function() {
        ThemeManager.draw(this, this.state);
    },

    toElement: function() {
        return this.element;
    }
});

var Button = new Class({

    Extends: [Widget],

    // class name should be inhertied somehow from parent, to get full class: 'art-widget art-button'
    className: 'art-button',

    initialize: function(options) {
        this.parent();
    }
});