You would need to create a custom control template. Here's an example you can adapt:
<!-- This defines the style/template for the up and down buttons -->
<Style x:Key="LuridButton" TargetType="RepeatButton">
<Setter Property="Focusable" Value="False" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="RepeatButton">
<Border BorderBrush="Lime" BorderThickness="2"
Background="Pink"
CornerRadius="4"
Padding="1,1.5,1.5,1.5" Name="Bd">
<ContentPresenter VerticalAlignment="Center" />
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter TargetName="Bd" Property="Background" Value="HotPink" />
</Trigger>
<Trigger Property="IsPressed" Value="True">
<Setter TargetName="Bd" Property="Background" Value="Red" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<!-- This defines the style/template for the spin control itself -->
<Style TargetType="ms:Spin" x:Key="Lurid">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ms:Spin">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="1" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<RepeatButton Command="{x:Static ms:SpinCommands.Increase}" Grid.Column="0"
Style="{StaticResource LuridButton}">
<Polygon Points="3,0 6,3 0,3" Fill="Black" SnapsToDevicePixels="True" />
</RepeatButton>
<RepeatButton Command="{x:Static ms:SpinCommands.Decrease}" Grid.Column="2"
Style="{StaticResource LuridButton}">
<Polygon Points="3,3 6,0 0,0" Fill="Black" SnapsToDevicePixels="True" />
</RepeatButton>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
And reference it as <ms:Spin Style="{StaticResource Lurid}" />.
