An integer value in WPF Resources?

To make that declaration you need to import the System namespace:

xmlns:sys="clr-namespace:System;assembly=mscorlib"

...

<sys:Int32 x:Key="MyValue">1234</sys:Int32>

Note: you will need to use a Double for most WPF properties instead of an Int32

Looks like the Assembly has changed updated for 2020:

xmlns:sys="clr-namespace:System;assembly=System.Runtime"

<sys:Int32 x:Key="MyValue">1234</sys:Int32>

For StrokeThickness it should be

<system:Double x:Key="ThemeIconStrokeThickness">1</system:Double>

Related