Hi there,
I know this thread is quite old, but just adding this in case anyone still runs into this 
If the highlight state isnāt working, itās usually because the styling is targeting a different class than what your current layout is using. The Product List element can have slightly different structures depending on how it was built or customized.
You can use this example as a reference:
If itās still not applying, Iād recommend inspecting the element in your browser and checking which class is being applied when the product is selected, then updating the CSS to match that.
Since this involves custom styling, it may need a bit of adjustment based on your specific setup.
Let me know if you need help identifying the correct class