Button BorderBrush won't change - wpf

Question

BorderBrush color of my button template does not change to Yellow. Please help.
<Style x:Key="someName" TargetType="{x:Type Button}">
<Setter Property="Background" Value="LightGreen"/>
<Setter Property="BorderBrush" Value="Yellow"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Border Background="{TemplateBinding Background}">
<ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="Green"/>
</Trigger>
</Style.Triggers>
</Style>

Answer 1

You have to bind the BorderBrush to the templated control via TemplateBinding so that it applies the value defined in your style. Additionally, you have to set add a BorderThickness of at least one the same way, otherwise there is no border.
<Style x:Key="someName" TargetType="{x:Type Button}">
<Setter Property="Background" Value="LightGreen"/>
<Setter Property="BorderBrush" Value="Yellow"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Border Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}">
<ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="Green"/>
</Trigger>
</Style.Triggers>
</Style>
As a general note, a control template defines all parts and states of a control and transitions between them. Your control template only defines the Mouse Over state apart from the normal state. Missing states may harm the user experience. You can find a list of all parts and states for a button in the documentation.
It is recommendable to extract the default control template or style for a control with Blend or Visual Studio first and adapting that. I have extracted the default style and merged it with your style. You just have to adapt the Disabled and Pressed colors.
<Style x:Key="ButtonStyle" TargetType="{x:Type Button}">
<Setter Property="FocusVisualStyle" Value="{StaticResource FocusVisual}"/>
<Setter Property="Background" Value="LightGreen"/>
<Setter Property="BorderBrush" Value="Yellow"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="HorizontalContentAlignment" Value="Center"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="Padding" Value="1"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Border x:Name="border" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" SnapsToDevicePixels="true">
<ContentPresenter x:Name="contentPresenter" Focusable="False" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsDefaulted" Value="true">
<Setter Property="BorderBrush" TargetName="border" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="Background" TargetName="border" Value="Green"/>
<Setter Property="BorderBrush" TargetName="border" Value="Yellow"/>
</Trigger>
<Trigger Property="IsPressed" Value="true">
<Setter Property="Background" TargetName="border" Value="#FFC4E5F6"/>
<Setter Property="BorderBrush" TargetName="border" Value="#FF2C628B"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Background" TargetName="border" Value="#FFF4F4F4"/>
<Setter Property="BorderBrush" TargetName="border" Value="#FFADB2B5"/>
<Setter Property="TextElement.Foreground" TargetName="contentPresenter" Value="#FF838383"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>