apprt/gtk: support light/dark mode change on the fly

src/apprt/gtk/App.zig

@@ -462,6 +462,7 @@ pub fn performAction(
         .equalize_splits => self.equalizeSplits(target),
         .goto_split => self.gotoSplit(target, value),
         .open_config => try configpkg.edit.open(self.core_app.alloc),
+        .config_change_conditional_state => self.configChangeConditionalState(target),
         .inspector => self.controlInspector(target, value),
         .desktop_notification => self.showDesktopNotification(target, value),
         .set_title => try self.setTitle(target, value),
@@ -487,7 +488,6 @@ pub fn performAction(
         .render_inspector,
         .renderer_health,
         .color_change,
-        .config_change_conditional_state,
         => log.warn("unimplemented action={}", .{action}),
     }
 }
@@ -817,6 +817,35 @@ fn showDesktopNotification(
     c.g_application_send_notification(g_app, n.body.ptr, notification);
 }
 
+fn configChangeConditionalState(
+    self: *App,
+    target: apprt.Target,
+) void {
+    const surface: *CoreSurface = switch (target) {
+        .app => return,
+        .surface => |v| v,
+    };
+
+    // Build our new configuration. We can free the memory
+    // immediately after because the surface will derive any
+    // values it needs to.
+    var new_config = self.config.changeConditionalState(
+        surface.config_conditional_state,
+    ) catch |err| {
+        // Not a big deal if we error... we just don't update
+        // the config. We log the error and move on.
+        log.warn("error changing config conditional state err={}", .{err});
+        return;
+    };
+    defer new_config.deinit();
+
+    // Update our surface.
+    surface.updateConfig(&new_config) catch |err| {
+        log.warn("error updating surface config for state change err={}", .{err});
+        return;
+    };
+}
+
 /// Reload the configuration. This should return the new configuration.
 /// The old value can be freed immediately at this point assuming a
 /// successful return.