Add CSS provider for thumbnail grid styling

Load application CSS at startup with rounded corners for thumbnail
frames and grid padding.

pixstrip-gtk/src/app.rs

@@ -137,7 +137,32 @@ fn setup_shortcuts(app: &adw::Application) {
     app.set_accels_for_action("win.show-shortcuts", &["<Control>question", "F1"]);
 }
 
+fn load_css() {
+    let provider = gtk::CssProvider::new();
+    provider.load_from_string(
+        r#"
+        .thumbnail-frame {
+            border-radius: 8px;
+            background: @card_bg_color;
+        }
+        .thumbnail-grid {
+            padding: 8px;
+        }
+        .thumbnail-check {
+            opacity: 0.9;
+        }
+        "#,
+    );
+    gtk::style_context_add_provider_for_display(
+        &gtk::gdk::Display::default().expect("Could not get default display"),
+        &provider,
+        gtk::STYLE_PROVIDER_PRIORITY_APPLICATION,
+    );
+}
+
 fn build_ui(app: &adw::Application) {
+    load_css();
+
     // Restore last-used wizard settings from session
     let sess = pixstrip_core::storage::SessionStore::new();
     let sess_state = sess.load().unwrap_or_default();