Overwrite the default Windows 10 style for xaml ui elements - wpf

Question

I´ve tried to change the IsMouseOver & IsFocused BorderBrush of a TextBox.
But it seems not to take affect, I always get this ugly standart blue color.
My style is inside a ResourceDictionary.
It´s working perfectly fine, except the problem with changing the BorderBrush.
<Style x:Key="TextBoxA" TargetType="TextBox" >
<Setter Property="Foreground" Value="#FFF0E6D2"/>
<Setter Property="FontSize" Value="15" />
<Setter Property="BorderBrush" Value="#FF785A28"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="Background" Value="#FF000306"/>
<Style.Triggers>
<Trigger Property="IsFocused" Value="True">
<Setter Property="Background" >
<Setter.Value>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#FF040B11" Offset="1"/>
<GradientStop Color="#FF11171B" Offset="0"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter Property="BorderBrush" >
<Setter.Value>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#FF785A28" Offset="1"/>
<GradientStop Color="#FFC8C36E" Offset="0"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
</Trigger>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="BorderBrush" Value="#FF785A28"/>
</Trigger>
</Style.Triggers>
</Style>

Answer 1

You need to modify the ControlTemplate. Right-click on the TextBox in design mode in Visual Studio and choose Edit Template->Edit a Copy to copy the default template into your XAML markup and then edit it as per your requirements:
<TextBox>
<TextBox.Style>
<Style TargetType="{x:Type TextBox}">
<Setter Property="Foreground" Value="#FFF0E6D2"/>
<Setter Property="FontSize" Value="15" />
<Setter Property="BorderBrush" Value="#FF785A28"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="Background" Value="#FF000306"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="KeyboardNavigation.TabNavigation" Value="None"/>
<Setter Property="HorizontalContentAlignment" Value="Left"/>
<Setter Property="FocusVisualStyle" Value="{x:Null}"/>
<Setter Property="AllowDrop" Value="true"/>
<Setter Property="ScrollViewer.PanningMode" Value="VerticalFirst"/>
<Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBox}">
<Border x:Name="border" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="True">
<ScrollViewer x:Name="PART_ContentHost" Focusable="false" HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden"/>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Opacity" TargetName="border" Value="0.56"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="BorderBrush" TargetName="border" Value="#FF785A28"/>
</Trigger>
<Trigger Property="IsKeyboardFocused" Value="true">
<Setter Property="Background" >
<Setter.Value>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#FF040B11" Offset="1"/>
<GradientStop Color="#FF11171B" Offset="0"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter Property="BorderBrush" >
<Setter.Value>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#FF785A28" Offset="1"/>
<GradientStop Color="#FFC8C36E" Offset="0"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsInactiveSelectionHighlightEnabled" Value="true"/>
<Condition Property="IsSelectionActive" Value="false"/>
</MultiTrigger.Conditions>
<Setter Property="SelectionBrush" Value="{DynamicResource {x:Static SystemColors.InactiveSelectionHighlightBrushKey}}"/>
</MultiTrigger>
</Style.Triggers>
</Style>
</TextBox.Style>
</TextBox>

Answer 2

Unless you edit the template of the control, you are pretty limited in what you can change. Here is the default template for the WPF TextBox, so change it to fit your desired style: https://learn.microsoft.com/en-us/dotnet/framework/wpf/controls/textbox-styles-and-templates