How to apply the theme resource changes after it has modified from the code behind

i have created a custom control by overriding textbox. in that control I have modified the resources collection, but it is not reflecting in the UI that is loaded earlier. Hence need to apply some logic to reapply the theme changes.

using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Controls;
using System.Collections.Generic;

namespace Sample
{
    public sealed class CustomTextBox : TextBox
    {
        public CustomTextBox()
        {
            this.DefaultStyleKey = typeof(TextBox);
        }

        #region IsMandatory

        public static readonly DependencyProperty IsMandatoryProperty = DependencyProperty.Register(nameof(IsMandatory), typeof(bool), typeof(CustomTextBox), new PropertyMetadata(false, OnIsMandatoryPropertyChanged));

        public bool IsMandatory
        {
            get { return (bool)GetValue(IsMandatoryProperty); }
            set { SetValue(IsMandatoryProperty, value); }
        }

        private static void OnIsMandatoryPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (e.OldValue != e.NewValue)
            {
                var control = d as CustomTextBox;
                bool isUpdated;

                if (control.IsMandatory)
                {
                    isUpdated = control.Resources.AddOrUpdate("TextControlHeaderForeground", "Red");
                }
                else
                {
                    isUpdated = control.Resources.AddOrUpdate("TextControlHeaderForeground", control.Resources.GetValueOrDefault("SystemControlForegroundBaseHighBrush"));
                }

                if (isUpdated)
                {
                    control.UpdateTheme();
                }
            }
        }


        #endregion

        public void UpdateTheme()
        {
            // need to update the UI to reflect the changes made in resources
        }
    }

    public static class Extensions
    {
        public static TValue GetValueOrDefault<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key)
        {
            return dict.TryGetValue(key, out var value) ? value : default(TValue);
        }

        public static bool AddOrUpdate<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TKey key, TValue value)
        {
            if (dictionary.ContainsKey(key))
            {
                if (!dictionary[key].Equals(value))
                {
                    dictionary[key] = value;
                    return true;
                }
            }
            else
            {
                dictionary.Add(key, value);
                return true;
            }

            return false;
        }
    }
}