feat(gallery): add style system demo

Author: mathiasbourgoin

example/gallery/dune

@@ -36,6 +36,7 @@
   braille_demo
   direct_page_demo
   box_widget_demo
+  style_system_demo
   textarea_demo
   focus_container_demo
   canvas_demo

example/gallery/launcher.ml

@@ -346,6 +346,13 @@ let demos =
           "demo_box_widget"
           (module Box_widget_demo.Page : Miaou.Core.Tui_page.PAGE_SIG);
     };
+    {
+      title = "Style System";
+      open_demo =
+        goto
+          "demo_style_system"
+          (module Style_system_demo.Page : Miaou.Core.Tui_page.PAGE_SIG);
+    };
     {
       title = "Textarea";
       open_demo =