some changes

cht/app.py

@@ -7,7 +7,7 @@ import gi
 gi.require_version("Gtk", "4.0")
 gi.require_version("Adw", "1")
 
-from gi.repository import Gtk, Adw, Gio, GLib
+from gi.repository import Gtk, Gdk, Adw, Gio, GLib
 
 from cht.config import APP_ID, APP_NAME
 from cht.window import ChtWindow
@@ -23,6 +23,15 @@ class ChtApp(Adw.Application):
     def do_activate(self):
         win = self.props.active_window
         if not win:
+            css = Gtk.CssProvider()
+            css.load_from_string(
+                ".frame-selected { border: 3px solid @accent_color; border-radius: 6px; }"
+            )
+            Gtk.StyleContext.add_provider_for_display(
+                Gdk.Display.get_default(),
+                css,
+                Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION,
+            )
             win = ChtWindow(application=self)
         win.present()