Thank you for writing.
When you are inheriting some of our controls and you want our theming mechanism to apply
the theme for it you should set it ThemeClassName
property. Consider the following example :
However, in your case you are inheriting element not control, so you should not set ThemeClassName
property but you should override the ThemeEffectiveType
property in your custom element. This will point our theming mechanism how to style your custom element.
Add the following code into your custom tree node:
I hope this helps.