我需要在Avalonia中将处于获焦状态的TextBox背景改为完全透明，该TextBox位于模板化的自定义用户控件内，且嵌套在Border中。目前未获焦时TextBox能保持透明，但获焦后会出现非透明背景，问题与Fluent Theme相关，且无法移除该主题包（其他组件依赖）。

尝试过的代码及现象 #

初始样式代码 #

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:controls="using:AIVA.CustomComponents"
        >
    
    <Style Selector="TextBox:focus-within">
        <Setter Property="Background" Value="{DynamicResource TransparentBrush}" />
        <Setter Property="BorderBrush" Value="{DynamicResource TransparentBrush}" />
        <Setter Property="BorderThickness" Value="0"></Setter>
        <Setter Property="Foreground" Value="Black" />
    </Style>
    
    <Design.PreviewWith>
        <Border Background="Aquamarine" Padding="100">
            <StackPanel>
                <controls:CustomInputField LabelText="ID" />
                <controls:CustomInputField/>
            </StackPanel>
        </Border>
        
    </Design.PreviewWith>

    <Style Selector="controls|CustomInputField">
        <Setter Property="Background" Value="Transparent"></Setter>
        <!-- Set Defaults -->
        <Setter Property="Template">
            <ControlTemplate>
                <Border
                    Width="350"
                    Height="25"
                    Margin="0,25,0,0"
                    HorizontalAlignment="Center"
                    VerticalAlignment="Center"
                    BorderBrush="Black"
                    BorderThickness="0,0,0,2">
                    <StackPanel Orientation="Horizontal">
                        <TextBlock
                            Width="100"
                            Height="26"
                            VerticalAlignment="Center"
                            FontFamily="Montserrat"
                            FontSize="18"
                            FontWeight="Medium"
                            Text="{TemplateBinding LabelText}"
                            TextAlignment="Left" />

                        <TextBox
                            x:Name="TextInput"
                            Width="250"
                            Height="26"
                            VerticalContentAlignment="Center"
                            Background="{DynamicResource TransparentBrush}"
                            BorderThickness="0,0,0,0"
                            FontSize="18"
                            FontWeight="Medium"
                            TextAlignment="Center"
                            Text="{TemplateBinding InputText}"
                            
                            >
                            
                        </TextBox>
                    </StackPanel>

                </Border>
            </ControlTemplate>
        </Setter>
    </Style>
</Styles>

现象：仅未获焦的TextBox保持透明背景

带类选择器的样式代码 #

<Style Selector="TextBox.CustomInput">
        <Setter Property="Background" Value="{DynamicResource TransparentBrush}"></Setter>
        <Setter Property="BorderBrush" Value="{DynamicResource TransparentBrush}"></Setter>
        
    </Style>
    <Style Selector="TextBox.CustomInput:focus-within">
        <Setter Property="Background" Value="{DynamicResource TransparentBrush}"></Setter>
        <Setter Property="BorderBrush" Value="{DynamicResource TransparentBrush}"></Setter>
        
    </Style>

现象：获焦后仍未实现透明

解决方案 #

由于Fluent Theme的TextBox模板在获焦时会通过内部资源或视觉状态触发器覆盖自定义样式，可通过以下几种方式解决：

方案1：使用高优先级选择器覆盖样式 #

针对自定义控件内的TextBox，使用更具体的选择器强制覆盖Fluent Theme的默认样式：

<Style Selector="controls|CustomInputField TextBox:focus-within">
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="BorderBrush" Value="Transparent" />
    <Setter Property="BorderThickness" Value="0" />
    <!-- 重写TextBox模板，移除内部默认背景 -->
    <Setter Property="Template">
        <ControlTemplate TargetType="TextBox">
            <Border Background="{TemplateBinding Background}"
                    BorderBrush="{TemplateBinding BorderBrush}"
                    BorderThickness="{TemplateBinding BorderThickness}">
                <ScrollViewer x:Name="PART_ContentHost"
                              Padding="{TemplateBinding Padding}"
                              Background="Transparent" />
            </Border>
        </ControlTemplate>
    </Setter>
</Style>

方案2：重写TextBox视觉状态 #

保留TextBox默认行为，仅修改获焦状态的背景值：

<Style Selector="TextBox:focus-within">
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="BorderBrush" Value="Transparent" />
    <!-- 强制覆盖视觉状态的背景设置 -->
    <Setter Property="VisualStateManager.VisualStateGroups">
        <VisualStateGroupList>
            <VisualStateGroup x:Name="CommonStates">
                <VisualState x:Name="Normal" />
                <VisualState x:Name="PointerOver" />
                <VisualState x:Name="Focused">
                    <VisualState.Setters>
                        <Setter Property="Background" Value="Transparent" />
                        <Setter Property="BorderBrush" Value="Transparent" />
                    </VisualState.Setters>
                </VisualState>
            </VisualStateGroup>
        </VisualStateGroupList>
    </Setter>
</Style>

方案3：覆盖Fluent Theme默认资源 #

直接替换主题中TextBox获焦时使用的背景资源，全局生效：

<Application.Resources>
    <SolidColorBrush x:Key="TextBoxFocusedBackground" Color="Transparent" />
    <SolidColorBrush x:Key="TextBoxFocusedBorderBrush" Color="Transparent" />
</Application.Resources>

内容的提问来源于stack exchange，提问作者Alexandro Cardona