Inject IConfiguration in controller to read the configuration value.

HomeController.cs

public class HomeController : Controller
{
    private readonly ILogger<HomeController> _logger;
    private readonly IConfiguration configuration;

    public HomeController(ILogger<HomeController> logger, IConfiguration configuration)
    {
        _logger = logger;
        this.configuration = configuration;
    }

    public IActionResult Index()
    {
        ViewData["Mode"] = configuration["Mode"];
        return View();
    }

    public IActionResult Privacy()
    {
        return View();
    }

    [ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
    public IActionResult Error()
    {
        return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
    }
}