要在禁用BorderBrush时更改TextBox，您需要覆盖默认的TextBox样式。

这是为您的案例编辑的默认TextBox样式，

    <Style TargetType="TextBox">
        <Setter Property="Width" Value="160"/>
        <Setter Property="Margin" Value="8"/>
        <Setter Property="IsEnabled" Value="False"/>
        <Setter Property="HorizontalAlignment" Value="Left"/>
        <Setter Property="BorderBrush" Value="Gray"/>
        <Setter Property="MinWidth" Value="{ThemeResource TextControlThemeMinWidth}" />
        <Setter Property="MinHeight" Value="{ThemeResource TextControlThemeMinHeight}" />
        <Setter Property="Foreground" Value="{ThemeResource SystemControlForegroundBaseHighBrush}" />
        <Setter Property="Background" Value="{ThemeResource SystemControlBackgroundAltHighBrush}" />
        <Setter Property="BorderBrush" Value="{ThemeResource SystemControlBackgroundBaseLowBrush}" />
        <Setter Property="SelectionHighlightColor" Value="{ThemeResource SystemControlHighlightAccentBrush}" />
        <Setter Property="BorderThickness" Value="1" />
        <Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}" />
        <Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}" />
        <Setter Property="ScrollViewer.HorizontalScrollMode" Value="Auto" />
        <Setter Property="ScrollViewer.VerticalScrollMode" Value="Auto" />
        <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Hidden" />
        <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Hidden" />
        <Setter Property="ScrollViewer.IsDeferredScrollingEnabled" Value="False" />
        <Setter Property="Padding" Value="{ThemeResource TextControlThemePadding}"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="TextBox">
                    <Grid>
                        <Grid.Resources>
                            <Style x:Name="DeleteButtonStyle" TargetType="Button">
                                <Setter Property="Template">
                                    <Setter.Value>
                                        <ControlTemplate TargetType="Button">
                                            <Grid x:Name="ButtonLayoutGrid" BorderBrush="{ThemeResource TextBoxButtonBorderThemeBrush}"
                                          BorderThickness="{TemplateBinding BorderThickness}"
                                          Background="{ThemeResource TextBoxButtonBackgroundThemeBrush}">
                                                <VisualStateManager.VisualStateGroups>
                                                    <VisualStateGroup x:Name="CommonStates">
                                                        <VisualState x:Name="Normal" />
                                                        <VisualState x:Name="PointerOver">
                                                            <Storyboard>
                                                                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="GlyphElement" Storyboard.TargetProperty="Foreground">
                                                                    <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAccentBrush}" />
                                                                </ObjectAnimationUsingKeyFrames>
                                                            </Storyboard>
                                                        </VisualState>
                                                        <VisualState x:Name="Pressed">
                                                            <Storyboard>
                                                                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ButtonLayoutGrid" Storyboard.TargetProperty="Background">
                                                                    <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAccentBrush}" />
                                                                </ObjectAnimationUsingKeyFrames>
                                                                <ObjectAnimationUsingKeyFrames Storyboard.TargetName="GlyphElement" Storyboard.TargetProperty="Foreground">
                                                                    <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAltChromeWhiteBrush}" />
                                                                </ObjectAnimationUsingKeyFrames>
                                                            </Storyboard>
                                                        </VisualState>
                                                        <VisualState x:Name="Disabled">
                                                            <Storyboard>
                                                                <DoubleAnimation Storyboard.TargetName="ButtonLayoutGrid"
                                                             Storyboard.TargetProperty="Opacity"
                                                             To="0"
                                                             Duration="0" />
                                                            </Storyboard>
                                                        </VisualState>
                                                    </VisualStateGroup>
                                                </VisualStateManager.VisualStateGroups>
                                                <TextBlock x:Name="GlyphElement"
                                                Foreground="{ThemeResource SystemControlForegroundChromeBlackMediumBrush}"
                                                VerticalAlignment="Center"
                                                HorizontalAlignment="Center"
                                                FontStyle="Normal"
                                                FontSize="12"
                                                Text="&#xE10A;"
                                                FontFamily="{ThemeResource SymbolThemeFontFamily}"
                                                AutomationProperties.AccessibilityView="Raw"/>
                                            </Grid>
                                        </ControlTemplate>
                                    </Setter.Value>
                                </Setter>
                            </Style>
                        </Grid.Resources>
                        <VisualStateManager.VisualStateGroups>
                            <VisualStateGroup x:Name="CommonStates">
                                <VisualState x:Name="Disabled">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="HeaderContentPresenter"
                                         Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="Black" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BackgroundElement"
                                       Storyboard.TargetProperty="Background">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="Gray" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement"
                                       Storyboard.TargetProperty="Background">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="Gray" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement"
                                       Storyboard.TargetProperty="BorderBrush">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="Gray" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement"
                                       Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="Black" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter"
                                       Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlDisabledChromeDisabledLowBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                                <VisualState x:Name="Normal" />
                                <VisualState x:Name="PointerOver">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightChromeAltLowBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BackgroundElement" Storyboard.TargetProperty="Opacity">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundHoverOpacity}" />
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                                <VisualState x:Name="Focused">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlPageTextChromeBlackMediumLowBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BackgroundElement" Storyboard.TargetProperty="Background">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlBackgroundChromeWhiteBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BackgroundElement" Storyboard.TargetProperty="Opacity">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundFocusedOpacity}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlHighlightAccentBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource SystemControlForegroundChromeBlackHighBrush}" />
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="RequestedTheme">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="Light" />
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                            </VisualStateGroup>
                            <VisualStateGroup x:Name="ButtonStates">
                                <VisualState x:Name="ButtonVisible">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="DeleteButton"
                                       Storyboard.TargetProperty="Visibility">
                                            <DiscreteObjectKeyFrame KeyTime="0">
                                                <DiscreteObjectKeyFrame.Value>
                                                    <Visibility>Visible</Visibility>
                                                </DiscreteObjectKeyFrame.Value>
                                            </DiscreteObjectKeyFrame>
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                                <VisualState x:Name="ButtonCollapsed" />
                            </VisualStateGroup>
                        </VisualStateManager.VisualStateGroups>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="*" />
                            <ColumnDefinition Width="Auto" />
                        </Grid.ColumnDefinitions>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto" />
                            <RowDefinition Height="*" />
                        </Grid.RowDefinitions>
                        <Border x:Name="BackgroundElement"
                        Grid.Row="1"
                        Background="{TemplateBinding Background}"
                        Margin="{TemplateBinding BorderThickness}"
                        Opacity="{ThemeResource TextControlBackgroundRestOpacity}"
                        Grid.ColumnSpan="2"
                        Grid.RowSpan="1"/>
                        <Border x:Name="BorderElement"
                        Grid.Row="1"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        Grid.ColumnSpan="2"
                        Grid.RowSpan="1"/>
                        <ContentPresenter x:Name="HeaderContentPresenter"
                          x:DeferLoadStrategy="Lazy"
                          Visibility="Collapsed"
                          Grid.Row="0"
                          Foreground="{ThemeResource SystemControlForegroundBaseHighBrush}"
                          Margin="0,8"
                          Grid.ColumnSpan="2"
                          Content="{TemplateBinding Header}"
                          ContentTemplate="{TemplateBinding HeaderTemplate}"
                          FontWeight="Normal" />
                        <ScrollViewer x:Name="ContentElement"
                          Grid.Row="1"
                          HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}"
                          HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
                          VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}"
                          VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}"
                          IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}"
                          IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}"
                          IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}"
                          Margin="{TemplateBinding BorderThickness}"
                          Padding="{TemplateBinding Padding}"
                          IsTabStop="False"
                          VerticalAlignment="Center"
                          AutomationProperties.AccessibilityView="Raw"
                          ZoomMode="Disabled" />
                        <ContentControl x:Name="PlaceholderTextContentPresenter"
                          Grid.Row="1"
                          Foreground="{ThemeResource SystemControlForegroundBaseMediumLowBrush}"
                          Margin="{TemplateBinding BorderThickness}"
                          Padding="{TemplateBinding Padding}"
                          IsTabStop="False"
                          Grid.ColumnSpan="2"
                          VerticalAlignment="Center"
                          Content="{TemplateBinding PlaceholderText}"
                          IsHitTestVisible="False"/>
                        <Button x:Name="DeleteButton"
                        Grid.Row="1"
                        Style="{StaticResource DeleteButtonStyle}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        Margin="{ThemeResource HelperButtonThemePadding}"
                        IsTabStop="False"
                        Grid.Column="1"
                        Visibility="Collapsed"
                        FontSize="{TemplateBinding FontSize}"
                        MinWidth="34"
                        VerticalAlignment="Stretch"/>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

在上面的代码中，将有一个名为“禁用”的VisualState，其中包含禁用文本框的属性。在这里BorderBrush的值更改为SystemControlBackgroundBaseLowBrush。