[GUI] Add dark theme (#388)

DiscordChatExporter.Gui/Theme.cs

@@ -0,0 +1,42 @@
+using MaterialDesignThemes.Wpf;
+using System.Windows.Media;
+
+namespace DiscordChatExporter.Gui
+{
+    public sealed class Theme
+    {
+        public static Theme Light { get; } = new Theme(new MaterialDesignLightTheme(), HexToColor.Convert("#343838"), HexToColor.Convert("#F9A825"));
+        public static Theme Dark { get; } = new Theme(new MaterialDesignDarkTheme(), HexToColor.Convert("#E8E8E8"), HexToColor.Convert("#F9A825"));
+
+        public static void SetCurrent(Theme theme)
+        {
+            var paletteHelper = new PaletteHelper();
+
+            var materialTheme = paletteHelper.GetTheme();
+            materialTheme.SetBaseTheme(theme.BaseTheme);
+            materialTheme.SetPrimaryColor(theme.PrimaryColor);
+            materialTheme.SetSecondaryColor(theme.SecondaryColor);
+
+            paletteHelper.SetTheme(materialTheme);
+        }
+
+        public Theme(IBaseTheme baseTheme, Color primaryColor, Color secondaryColor)
+        {
+            BaseTheme = baseTheme;
+            PrimaryColor = primaryColor;
+            SecondaryColor = secondaryColor;
+        }
+
+        public IBaseTheme BaseTheme { get; }
+        public Color PrimaryColor { get; }
+        public Color SecondaryColor { get; }
+
+        class HexToColor
+        {
+            public static Color Convert(string hex)
+            {
+                return (Color)ColorConverter.ConvertFromString(hex);
+            }
+        }
+    }
+}