CssStyleExtension.cs

using System;
using System.Linq;
using System.Windows;
using System.Windows.Markup;

namespace WpfCssStyle
{
    [MarkupExtensionReturnType(typeof(Style))]
    public class CssStyleExtension : MarkupExtension
    {
        /// <summary>
        ///     Set space-separated style names i.e. "size16 gray verdana".
        /// </summary>
        public string Names { private get; set; }

        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            return Names.Split(new[] { ' ' })
                        .Select(x => Application.Current.TryFindResource(x)
                            as Style)
                        .Aggregate(new Style(), RecursivelyMergeStyles);
        }

        private static Style RecursivelyMergeStyles(Style accumulator, Style next)
        {
            if (accumulator == null || next == null)
                return accumulator;

            if (next.BasedOn != null)
                RecursivelyMergeStyles(accumulator, next.BasedOn);

            MergeStyle(accumulator, next);

            return accumulator;
        }

        private static void MergeStyle(Style targetStyle, Style sourceStyle)
        {
            if (targetStyle == null || sourceStyle == null)
            {
                return;
            }

            targetStyle.TargetType = sourceStyle.TargetType;

            // Merge the Setters...
            foreach (var setter in sourceStyle.Setters)
                targetStyle.Setters.Add(setter);

            // Merge the Triggers...
            foreach (var trigger in sourceStyle.Triggers)
                targetStyle.Triggers.Add(trigger);
        }
    }
}