WPF custom control style only applied on certain computers

Question

I've been writing a WPF program at work that uses a custom AnimatedScrollViewercontrol, inheriting from ScrollViewer. It has the following default template:

<Style TargetType="{x:Type Controls:AnimatedScrollViewer}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type Controls:AnimatedScrollViewer}">
                <Grid x:Name="PART_LayoutRoot">
                    <Grid.RowDefinitions>
                        <RowDefinition Height="*" />
                        <RowDefinition Height="Auto" />
                    </Grid.RowDefinitions>

                    <ScrollContentPresenter x:Name="PART_ScrollContentPresenter" />

                    <Grid Grid.Row="1">
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="*" />
                            <ColumnDefinition Width="Auto" />
                        </Grid.ColumnDefinitions>

                        <ScrollBar x:Name="PART_HorizontalScrollBar"
                                   Orientation="Horizontal"
                                   IsEnabled="{TemplateBinding IsPaused}"
                                   Value="{TemplateBinding HorizontalOffset}"
                                   Maximum="{TemplateBinding ScrollableWidth}"
                                   ViewportSize="{TemplateBinding ViewportWidth}"
                                   Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}" />

                        <StackPanel Grid.Column="1">
                            <Image Source="{StaticResource LockedImage}"
                                   Visibility="{TemplateBinding IsPlaying, Converter={StaticResource boolToVisConverter}}"/>
                            <Image Source="{StaticResource UnlockedImage}"
                                   Visibility="{TemplateBinding IsPaused, Converter={StaticResource boolToVisConverter}}"/>
                        </StackPanel>
                    </Grid>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

I'm setting this as the default style using:

[...]
public class AnimatedScrollViewer : ScrollViewer
{
    static AnimatedScrollViewer()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(AnimatedScrollViewer), new FrameworkPropertyMetadata(typeof(AnimatedScrollViewer)));
    }
[...]

The style is being referenced in the below Generic.xaml dictionary, which in turn is referenced as a MergedDictionary in App.xaml:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="ConvertersDictionary.xaml" />
        <ResourceDictionary Source="pack://application:,,,/JamesWright.Utils.EndpointCaller.Views;component/Themes/EndpointStyles.xaml" />
        <ResourceDictionary Source="pack://application:,,,/JamesWright.Utils.EndpointCaller.Views;component/Themes/ImageResources.xaml" />
        <ResourceDictionary Source="pack://application:,,,/JamesWright.Utils.EndpointCaller.Views;component/Themes/Resources.xaml" />
        <ResourceDictionary Source="pack://application:,,,/JamesWright.Utils.EndpointCaller.Views;component/Controls/AnimatedScrollViewer.xaml" />
    </ResourceDictionary.MergedDictionaries>
</ResourceDictionary>

What's really strange is that the style is only applied to the control when the program runs on my own machine and a Windows 8.1 virtual machine, while it isn't applied on my colleague's computer and a team machine. I know that the style isn't applied because the associated ScrollBar should only be enabled when 'IsPaused' is true. On my colleague's machine, however, it's always enabled. Also, the images that I've specified in the template don't appear as a result.

Are there any known reasons why a Style may not be applied on certain machines? I've decompiled the app and the resources are referencing the correct files. Any help is appreciated, as we've been pulling our hair out all day :P

UPDATE: I've downloaded it to my Windows 8 laptop and the style is applied correctly.

UPDATE #2: I attempted to use relative paths as opposed to pack syntax with no success

Answer 1

After reading the accepted answer to this question, I believe the issue is down to different system themes, although the settings between my machine and the team computer seem the same. If I apply the style explicitly using the Style attribute, then the correct behaviour is observed across all machines.

I am going to further investigate the relationship between Windows themes and default WPF control templates and I'll update this answer if I find anything useful.