Filter screenshots by style if prefer dark mode (elementary#2210)

If the environment (if any) does not match the prefer dark setup.
Won't consider null environments.

src/Views/AppInfoView.vala

@@ -942,7 +942,21 @@ public class AppCenter.Views.AppInfoView : Adw.NavigationPage {
             var scale = get_scale_factor ();
             var min_screenshot_width = MAX_WIDTH * scale;
 
+            var prefer_dark_theme = Gtk.Settings.get_default ().gtk_application_prefer_dark_theme;
             screenshots.foreach ((screenshot) => {
+                var environment_id = screenshot.get_environment ();
+                if (prefer_dark_theme && environment_id != null) {                    
+                    var environment_split = environment_id.split (":", 2);
+                    if (environment_split.length != 2) {
+                        return;
+                    }
+
+                    var color_scheme = AppStream.ColorSchemeKind.from_string (environment_split[1]);
+                    if (color_scheme != AppStream.ColorSchemeKind.DARK) {
+                        return;
+                    }
+                }
+
                 AppStream.Image? best_image = null;
                 screenshot.get_images ().foreach ((image) => {
                     // Image is better than no image