ThemeResource in Style isn't used when I build my UWP app - xaml

Question

I am trying to make a rectangle that changes color after a tapped event. It should cycle between a disabled mode (gray) and the accent color. I am trying to do this using styles. When I use (a shade of) the accent color in the XAML code {ThemeResource SystemControlHighlightAltListAccentHighBrush} it correctly shows the accent color. It also shows the correct colors in the designer (Visual Studio). However, if I use the theme colors in a resource dictionary, it replaces them by grayish or no colors in my app (after build). This is my ResourceDictionary:
<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:I_have_never">
<!--Disabled-->
<Style TargetType="RelativePanel" x:Key="Disabled">
<Setter Property="Tag">
<Setter.Value>0</Setter.Value>
</Setter>
<Setter Property="Margin">
<Setter.Value>30,15,30,15</Setter.Value>
</Setter>
<Setter Property="Background">
<Setter.Value>{ThemeResource SystemControlBackgroundListLowBrush} </Setter.Value>
</Setter>
</Style>
<!--Enabled-->
<Style TargetType="RelativePanel" x:Key="Enabled">
<Setter Property="Tag">
<Setter.Value>2</Setter.Value>
</Setter>
<Setter Property="Margin">
<Setter.Value>30,15,30,15</Setter.Value>
</Setter>
<Setter Property="Background">
<Setter.Value>{ThemeResource SystemControlHighlightAltListAccentHighBrush}</Setter.Value>
</Setter>
</Style>
</ResourceDictionary>
This is my XAML code:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="150" />
<RowDefinition />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition />
</Grid.ColumnDefinitions>
<RelativePanel Name="Category1"
Grid.Row="0"
Grid.Column="0"
Tapped="ToggleStatus"
Style="{StaticResource Enabled}">
</RelativePanel>
</Grid>
And this is my C# code to toggle between the styles:
private void ToggleStatus(object sender, RoutedEventArgs e)
{
RelativePanel panel = sender as RelativePanel;
if ((string)panel.Tag == "0")
{
panel.Style = (Style)App.Current.Resources["Enabled"]; // Tag = 1
}
else if ((string)panel.Tag == "1")
{
panel.Style = (Style)App.Current.Resources["Disabled"]; // Tag = 2
}
}
In the design view this works great. It nicely shows the grey or accent color backgrounds. However, when I build this, no colors are shown.
It does show colors as long as I use real colors (e.g. Gray) instead of the ThemeResource. It also works if I use the ThemeResource straight in the XAML code (no styles). Why does it only work in the designer? Why does it not work if I use the ThemeResource in the styles? And how could I fix this?

Answer 1

Why does it not work if I use the ThemeResource in the styles? And how could I fix this?
You are using XAML Property Element Usage Syntax to set the extensionUsage (i.e. {ThemeResource} or {StaticResource}) to Setter.Value property in the styles:
<Setter ...>
<Setter.Value>
objectValue
</Setter.Value>
</Setter>
According to the Syntax part of the official documentation Setter.Value property, when setting an extensionUsage (i.e. {ThemeResource} or {StaticResource}) to Setter.Value property in the styles, we should use XAML Attribute Usage Syntax instead of using XAML Property Element Usage Syntax:
<Setter Property="propertyName" Value="extensionUsage"/>
So you should use the following code to set the Background with {ThemeResource} in the styles:
<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:ThemeResourceInStyle">
<!--Disabled-->
<Style TargetType="RelativePanel" x:Key="Disabled">
<Setter Property="Tag">
<Setter.Value>0</Setter.Value>
</Setter>
<Setter Property="Margin">
<Setter.Value>30,15,30,15</Setter.Value>
</Setter>
<!--use XAML Attribute Usage Syntax to set extensionUsage -->
<Setter Property="Background" Value="{ThemeResource SystemControlBackgroundListLowBrush}" />
</Style>
<!--Enabled-->
<Style TargetType="RelativePanel" x:Key="Enabled">
<Setter Property="Tag">
<Setter.Value>1</Setter.Value>
</Setter>
<Setter Property="Margin">
<Setter.Value>30,15,30,15</Setter.Value>
</Setter>
<!--use XAML Attribute Usage Syntax to set extensionUsage -->
<Setter Property="Background" Value="{ThemeResource SystemControlHighlightAltListAccentHighBrush}" />
</Style>
</ResourceDictionary>