Your VisualBrush is designed to be assigned to an object that accepts a Brush, like a Background not a Style like you are attempting to. I would suggest that you do some thing like this.(This example will assign the same VisualBrush
to every TextBox
if you need to be able to pick and choose your brushes then I would suggest that you add back the x:Key
to your style and assign the Styles individually to your TextBox's)
<Window x:Class="WpfApplication4.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="400" Width="400">
<Window.Resources>
<VisualBrush x:Key="myVisualBrush">
<VisualBrush.Visual>
<Canvas>
<Path Fill="#FF231F20" Stretch="Fill" Width="12.69" Height="14.477" Canvas.Left="652.196" Canvas.Top="88.61" Data="F1M617.2051,52.7275C616.7281,52.4525,616.3391,52.6775,616.3391,53.2275L616.3391,66.4695C616.3391,67.0195,616.7281,67.2445,617.2051,66.9695L628.6721,60.3485C629.1481,60.0735,629.1481,59.6235,628.6721,59.3485z"/>
</Canvas>
</VisualBrush.Visual>
</VisualBrush>
<Style TargetType="{x:Type TextBox}">
<Setter Property="Background" Value="{StaticResource myVisualBrush}"/>
</Style>
<Style x:Key="myPathStyle" TargetType="{x:Type Path}">
<Setter Property="Fill" Value="{StaticResource myVisualBrush}"/>
</Style>
</Window.Resources>
<Grid>
<TextBox x:Name="MyTextBox" />
<Path Style="{StaticResource myPathStyle}" x:Name="myPath" >
<Path.Data>
<RectangleGeometry Rect="100,100,100,100"/>
</Path.Data>
</Path>
</Grid>
</Window>