add missing namespace to App.razor for Blazor WebApp

templates/app/aspnet-core/src/MyCompanyName.MyProjectName.Blazor.WebApp/Components/App.razor

@@ -2,6 +2,7 @@
 @using System.Globalization
 @using Microsoft.Extensions.Hosting
 @using Volo.Abp.AspNetCore.Components.Server.MudBlazorBasicTheme.Bundling
+@using Volo.Abp.AspNetCore.Components.Web.Theming.MudBlazor.Bundling
 @inject IHostEnvironment Env
 @{
     var rtl = CultureHelper.IsRtl ? "rtl" : string.Empty;