Trying to add icon sets to a theme option in my app.

Hello everyone!
I have a theming option in my app, that is not perfect, but functions well enough for my purpose, at least that's what I thought until I started making the light and dark themes and realized I needed to make a light and dark icons as well... Only problem is my icons are statically coded... So I figured, why not use the same <style ... tags for the icons as well?
Here is what I have in the code.
Here's a snippet from my theme code.

<Style x:Key="ic_Add" TargetType="{x:Type ToolbarItem}">
    <Setter Property="IconImageSource" Value="ic_plus_26px.png" />
</Style>

<Color x:Key="primary-title-color">#919191</Color>
<Color x:Key="NavigationPrimary">#6F6F6F</Color>
<Color x:Key="AppBackgroundColor">#373739</Color>

and when I attempt to implement it...

<ToolbarItem
        x:Name="btnAddLoad"
        Clicked="AddLoad_Clicked"
        StyleId="{StaticResource Key=ic_Add}"
        Text="Add" />

My problem is that the icon is not showing up... I've tried using the implementation as both StyleId and ClassId both with the same results...

Any ideas or recommendations?

Thanks!