Fix transparent popover backgrounds

Some GTK themes (and newer GTK versions) leave popover surfaces
transparent. Add the canonical 'menu' class to both popovers and a
theme-aware CSS override so the difficulty/leaderboard menus always
draw an opaque background.

src/window.cpp

@@ -21,6 +21,15 @@ MainWindow::MainWindow() {
     setup_header_bar();
     setup_board();
 
+    // Popovers: some GTK themes (and newer GTK versions) leave the popover
+    // surface transparent unless the content opts in. Guarantee an opaque,
+    // theme-aware background.
+    auto popover_css = Gtk::CssProvider::create();
+    popover_css->load_from_data(
+        "popover.background > contents { background-color: @theme_bg_color; }");
+    Gtk::StyleContext::add_provider_for_display(
+        get_display(), popover_css, GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+
     // Keyboard shortcuts
     auto key_controller = Gtk::EventControllerKey::create();
     key_controller->signal_key_pressed().connect([this](guint keyval, guint /*keycode*/, Gdk::ModifierType mods) {
@@ -116,6 +125,7 @@ void MainWindow::setup_header_bar() {
 
     btn_difficulty_.set_popover(menu_difficulty_);
     menu_difficulty_.set_child(box_difficulty_);
+    menu_difficulty_.add_css_class("menu");
     header_bar_.pack_start(btn_difficulty_);
 
     // Leaderboard
@@ -125,6 +135,7 @@ void MainWindow::setup_header_bar() {
     box_leaderboard_.set_spacing(4);
     btn_leaderboard_.set_popover(menu_leaderboard_);
     menu_leaderboard_.set_child(box_leaderboard_);
+    menu_leaderboard_.add_css_class("menu");
     menu_leaderboard_.signal_show().connect(sigc::mem_fun(*this, &MainWindow::refresh_leaderboard));
     header_bar_.pack_start(btn_leaderboard_);