<Window x:Class="WpfApp1.MainWindow"
          xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
          xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
          xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
          xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
          xmlns:local="clr-namespace:WpfApp1"
          mc:Ignorable="d"
          Title="MainWindow" Height="450" Width="800">
     <Grid>

          <TextBlock
               Text="Good morning!"
               Background="{StaticResource MyColorForBooks}"
               FontSize="30"
               HorizontalAlignment="Center" VerticalAlignment="Center" />

     </Grid>
</Window>

class MyUtils
{
     public static void ChangeColorBrushInResourceDictionary(ResourceDictionary resourceDictionary, string elemName, Color newColor)
     {
          resourceDictionary.BeginInit();
          var newBrush = new SolidColorBrush(newColor);
          resourceDictionary[elemName] = newBrush;
          resourceDictionary.EndInit();
     }
}

<Window x:Class="WpfApp1.MainWindow"
          xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
          xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
          xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
          xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
          xmlns:local="clr-namespace:WpfApp1"
          mc:Ignorable="d"
          Title="MainWindow" Height="450" Width="800">

     <Window.Resources>
          <ResourceDictionary x:Name="myResourceDictionary1"
               xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
               xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

               <SolidColorBrush
                    x:Key="MyColorForBooks"
                    Color="Green" />

          </ResourceDictionary>
     </Window.Resources>
         
     <Grid>

          <TextBlock
               Text="Good morning!"
               Background="{DynamicResource MyColorForBooks}"
               FontSize="30"
               HorizontalAlignment="Center" VerticalAlignment="Center" />

     </Grid>
</Window>

using System.Windows;
using System.Windows.Media;
using System.Timers;

namespace WpfApp1
{
     class MyUtils
     {
          public static void ChangeColorBrushInResourceDictionary(ResourceDictionary resourceDictionary, string elemName, Color newColor)
          {
               resourceDictionary.BeginInit();
               var newBrush = new SolidColorBrush(newColor);
               resourceDictionary[elemName] = newBrush;
               resourceDictionary.EndInit();
          }
     }

     public partial class MainWindow : Window
     {
          private readonly System.Timers.Timer _timer1;
          public MainWindow()
          {
               InitializeComponent();

               _timer1 = new System.Timers.Timer();
               _timer1.Interval = 5000; /*frequeny each 5 seconds*/
               _timer1.Elapsed += OnTimer;
               _timer1.Start();
          }

          void OnTimer(object sender, ElapsedEventArgs e)
          {
               _timer1.Stop();

               Dispatcher.Invoke(() => {
                    MyUtils.ChangeColorBrushInResourceDictionary(myResourceDictionary1, "MyColorForBooks", Color.FromRgb(0, 0, 255));
               });
          }
     }
}