ComboBox highlighting - wpf

Question

I have a problem with my combobox I'm trying to customize. It is in a UserControl, and i want its BorderBrush property to change from Transparent to White when the mouse is over it (fade in/out would be bonus).
But I can't seem to get the proper Trigger syntax to do so... now I feel confused and I'm probably missing something obvious here.
Here is the combobox in question:
<ComboBox x:Name="comboEmiCategories" ItemsSource="{Binding}" Background="Transparent" Height="15px" Width="30px" BorderBrush="Transparent" Padding="-2">
<ComboBox.Resources>
<sys:Double x:Key="{x:Static SystemParameters.VerticalScrollBarWidthKey}">0</sys:Double>
</ComboBox.Resources>
<ComboBox.ItemTemplate>
<DataTemplate>
<Image Source="{Binding}" Stretch="UniformToFill" Height="15px" Width="30px" Margin="0" />
</DataTemplate>
</ComboBox.ItemTemplate>
<ComboBox.Template>
<ControlTemplate>
<ControlTemplate.Triggers>
<Trigger Property="ComboBox.IsMouseOver" Value="True">
<Setter Property="ComboBox.BorderBrush" Value="White" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</ComboBox.Template>
</ComboBox>
The trigger isn't working, even worse actually, if i don't comment out the whole ComboBox.Template part, the control disappears.
The main goal would be to have a ComboBox that stacks images and allows the user to select one out of a list, with nothing else than those images shown.
Thanks.
EDIT:
Mario's solution of putting it within a style works, but is it the only way to do this?

Answer 1

Try to place the below xaml within the Resources section of your window/usercontrol.
<Style x:Name="cbStyle" TargetType="ComboBox">
<Setter Property="BorderBrush" Value="Transparent" />
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="BorderBrush" Value="White" />
</Trigger>
</Style.Triggers>
</Style>
Of course you must refer this style in the ComboBox element. Also cut anything within the ControlTemplate, which is useless.
EDIT: your ComboBox section should look as follows:
<ComboBox x:Name="comboEmiCategories" ItemsSource="{Binding}" Height="15px" Width="30px" Style="{StaticResource cbStyle}" Padding="-2">
<ComboBox.Resources>
<sys:Double x:Key="{x:Static SystemParameters.VerticalScrollBarWidthKey}">0</sys:Double>
</ComboBox.Resources>
<ComboBox.ItemTemplate>
<DataTemplate>
<Image Source="{Binding}" Stretch="UniformToFill" Height="15px" Width="30px" Margin="0" />
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
Also put the Style declatarion in your UserControl.