introduce theme files and handle in notification popup

ntfysh_client/Themes/DarkModeTheme.cs

@@ -0,0 +1,12 @@
+using System.Drawing;
+
+namespace ntfysh_client.Themes
+{
+    internal class DarkModeTheme: BaseTheme
+    {
+        public override Color BackgroundColor { get => SystemColors.ControlDark; }
+        public override Color ControlBackGroundColor { get => Color.Black; }
+        public override Color ControlMouseOverBackgroundColor { get => Color.Silver; }
+        public override Color ForegroundColor { get => SystemColors.ControlLight; }
+    }
+}