Probably you use version below 2.0.0. This problem was solved with version 2.0.0.
Version 2.0.0 you can download here.
Below is code snippet when you can see that in version below 2.0.0 is missing binding to template BorderBrush (BorderBrush="{TemplateBinding BorderBrush}"
).
Version 2.0.0 implementation (source code):
<Style x:Key="NumericUpDown" TargetType="{x:Type prim:InputBase}">
<Setter Property="Background" Value="{DynamicResource {x:Static themes:ResourceKeys.ControlNormalBackgroundKey}}" />
<Setter Property="BorderBrush" Value="{DynamicResource {x:Static themes:ResourceKeys.ControlNormalBorderKey}}" />
<Setter Property="BorderThickness" Value="1" />
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.WindowTextBrushKey}}" />
<Setter Property="HorizontalContentAlignment" Value="Right" />
<Setter Property="IsTabStop" Value="False" />
<Setter Property="VerticalContentAlignment" Value="Center" />
<Setter Property="TextAlignment" Value="Right" />
<Setter Property="WatermarkTemplate" Value="{StaticResource DefaultWatermarkTemplate}" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Control">
<local:ButtonSpinner x:Name="PART_Spinner"
IsTabStop="False"
Background="{TemplateBinding Background}"
BorderThickness="{TemplateBinding BorderThickness}"
BorderBrush="{TemplateBinding BorderBrush}"
AllowSpin="{Binding AllowSpin, RelativeSource={RelativeSource TemplatedParent}}"
ShowButtonSpinner="{Binding ShowButtonSpinner, RelativeSource={RelativeSource TemplatedParent}}">
...
Version 1.9.0 implementation (source code):
<Style x:Key="NumericUpDown" TargetType="{x:Type prim:InputBase}">
<Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.WindowBrushKey}}" />
<Setter Property="BorderThickness" Value="1" />
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.WindowTextBrushKey}}" />
<Setter Property="HorizontalContentAlignment" Value="Right" />
<Setter Property="IsTabStop" Value="False" />
<Setter Property="VerticalContentAlignment" Value="Center" />
<Setter Property="TextAlignment" Value="Right" />
<Setter Property="WatermarkTemplate" Value="{StaticResource DefaultWatermarkTemplate}" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Control">
<local:ButtonSpinner x:Name="PART_Spinner"
IsTabStop="False"
Background="{TemplateBinding Background}"
BorderThickness="{TemplateBinding BorderThickness}"
AllowSpin="{Binding AllowSpin, RelativeSource={RelativeSource TemplatedParent}}"
ShowButtonSpinner="{Binding ShowButtonSpinner, RelativeSource={RelativeSource TemplatedParent}}">
...