iron-overlay-backdrop style not being applied

[nate...@gmail.com]

I have a custom element which has an iron-overlay-backdrop as a child element (inside a paper-dialog). I am trying to increase the fade time (which I can do if I can edit the css in chrome). When I try and apply changes to the style --iron-overlay-backdrop the changes are not being applied in the child. What I have is:

<style>

:host {

    --iron-overlay-backdrop: {transition: opacity 5s;};

}

I have also tried something simpler with --iron-overlay-backdrop-opacity: 0.8 which does not work either. For some reason the style is not being applied. Does anyone know what I am doing wrong, or how I can get the desired behavior of a longer transition time?

[bal...@hockerill.com]