I've tried to follow:

I created two files

AzureTextBoxTheme.cs

public class AzureTextBoxTheme : ThemeBase
  {

    protected override void ConfigureControlMappings()
    {
      // Get the full name of the assembly where the theme's resource dictionaries are placed
      string assemblyFullName = Assembly.GetExecutingAssembly().FullName;

      Mappings.Add("System.Windows.Controls.TextBox",
               BuildLocationString(assemblyFullName, @"\AzureTextBoxTheme.xaml"));
    }

  }
}

AzureTextBoxTheme.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:SapQmWp.Themes">

  <Style TargetType="{x:Type TextBox}">
    <Setter Property="Background" Value="BlanchedAlmond"/>
  </Style>
    
</ResourceDictionary>

And in the MainWindow.cs:

public partial class MainWindow : XamRibbonWindow
  {
    public MainWindow()
    {
      InitializeComponent();
      ThemeManager.RegisterControl(typeof(TextBox));
      ThemeManager.ApplicationTheme = new Office2013Theme();
      
    }
  }

And in the xaml file, where I need to change the theme:

          <ig:ThemeManager.Theme>
            <themes:AzureTextBoxTheme />
          </ig:ThemeManager.Theme>
          <TextBox Grid.Column="0" Margin="{StaticResource MarginTen}" IsReadOnly="True" IsHitTestVisible="False"
                   Text="{Binding Order}" /> 

But I've got an exception:

{"Cannot locate resource 'azuretextboxtheme.xaml'."}