Textbox background color not applying from Style - wpf

Question

My textbox is in WPF User control and style is being applied as:
<ResourceDictionary Source="pack://application:,,,/MyStyles1;component/Themes/MyTheme.xaml"/>
Style is as follows:
<Style x:Key="OutputTextBoxStyle" TargetType="TextBox">
<Setter Property="FontSize" Value="15" />
<Setter Property="Background" Value="Green" />
<Setter Property="Foreground" Value="Blue" />
<Setter Property="BorderBrush" Value="Red" />
<Setter Property="BorderThickness" Value="15" />
<Setter Property="Padding" Value="2" />
</Style>
It is noted that all properties(e.g. Foreground, BorderBrush, BorderThickness etc) are working as required. But TextBox Background is not applied.
Please assist.

Answer 1

There are two ways of setting this style to text boxes:
Explicitly using:
<TextBox Style="{StaticResource OutputTextBoxStyle}" />
This is the usual way when only a few text boxes in the project need this style applying to them.
Implicitly using implicit styles. To get implicit styles working your definition shouldn't contain an x:Key definition.
So you either add a second definition like this:
<Style TargetType="TextBox"
BasedOn="{StaticResource OutputTextBoxStyle}" />
or remove the x:Key definition from your existing style.
You'll need to do the former if you ever want to explicitly reference the style from somewhere else.
You can also override a specific part of the style by specifying it:
<TextBox Style="{StaticResource OutputTextBoxStyle}"
Background="{Aqua}" />
or
<TextBox Background="{Aqua}" />

Answer 2

EDIT: I've only now noticed that you discussed this with Chris through comments, he pretty much asked you what I asked below.
Works with me:
<Window.Resources>
<Style x:Key="OutputTextBoxStyle" TargetType="TextBox">
<Setter Property="FontSize" Value="15" />
<Setter Property="Background" Value="Green" />
<Setter Property="Foreground" Value="Blue" />
<Setter Property="BorderBrush" Value="Red" />
<Setter Property="BorderThickness" Value="15" />
<Setter Property="Padding" Value="2" />
</Style>
</Window.Resources>
<Grid>
<TextBox Text="123123" HorizontalAlignment="Left" VerticalAlignment="Top" Style="{DynamicResource OutputTextBoxStyle}"/>
</Grid>
How does your TextBox actually look like (XAML)?
I suspect you've set the Background Property which is overriding the value that's inside your Style.