I built a quick and dirty theme browser, so that you can visually see what keys are available and what their values are.  I figured I'd share the code.  It creates a button with the background set to each brush.
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using ActiproSoftware.Windows.Themes;
namespace Vector
{
    public partial class TestWindow
    {
        public TestWindow()
        {
            this.InitializeComponent();
            foreach (string group in ThemeManager.GetGroupsForTheme(ThemeManager.CurrentTheme))
            {
                var dictionary = ThemeManager.GetResourceDictionary(ThemeManager.CurrentTheme, group);
                foreach (var merged in dictionary.MergedDictionaries)
                {
                    foreach (var item in merged.Keys)
                    {
                        var key = item as ResourceKey;
                        if (key != null)
                            Target.Children.Add(new Button() { Background = dictionary[key] as Brush, Content = key.ToString() });
                    }
                }
            }
        }
    }
}