add theme function. And provide two themes default.

WinAlfred/SettingWindow.xaml.cs

@@ -0,0 +1,46 @@
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Windows;
+using System.Windows.Controls;
+using WinAlfred.Helper;
+
+namespace WinAlfred
+{
+    public partial class SettingWidow : Window
+    {
+        private MainWindow mainWindow;
+
+        public SettingWidow(MainWindow mainWindow)
+        {
+            this.mainWindow = mainWindow;
+            InitializeComponent();
+            Loaded += Setting_Loaded;
+        }
+
+        private void Setting_Loaded(object sender, RoutedEventArgs e)
+        {
+            foreach (string theme in LoadAvailableThemes())
+            {
+                string themeName = theme.Substring(theme.LastIndexOf('\\') + 1).Replace(".xaml", "");
+                themeComboBox.Items.Add(themeName);
+            }
+
+            themeComboBox.SelectedItem = Settings.Instance.Theme;
+        }
+
+        private List<string> LoadAvailableThemes()
+        {
+            string themePath = Directory.GetCurrentDirectory() + "\\Themes\\";
+            return Directory.GetFiles(themePath).Where(filePath => filePath.EndsWith(".xaml")).ToList();
+        }
+
+        private void ThemeComboBox_OnSelectionChanged(object sender, SelectionChangedEventArgs e)
+        {
+            string themeName = themeComboBox.SelectedItem.ToString();
+            mainWindow.ChangeStyles(themeName);
+            Settings.Instance.Theme = themeName;
+            Settings.Instance.SaveSettings();
+        }
+    }
+}