PolymerJS: Allow custom elements to be styled with CSS variables

After we created a custom element, we learnt how to encapsulate its styles, style its host element and style its injected content.

But a custom element owner should specify what internal styles should be specified by the user.

PolymerJS 1.0 uses a basic shim for CSS variables for this.

In the element's local DOM, you'd surround your content in a tag with a class name <span class="aclass">styled content</span> for example.

Then in the style tag above the template, you'd specify that class, aclass in this case, with something like background-color: var(--an-ele-bgname, pink).

The above says there'll be a variable called --an-ele-bgname that should be used for the background-color of the class, or use pink if it doesn't exist.

Then in your index.html, ensure the style tag is extended by an is="custom-style", ensure the css selector that specifies our custom element specifies the CSS var named above:

<style is="custom-style">
    an-ele {
        --an-ele-bgname: red;


  1. If you specify the custom element from within another custom element, you don't need to extend style with is="custom=style".

Full source: index.html

        <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
        <script src="bower_components/webcomponentsjs/webcomponents-lite.min.js"></script>
        <link rel="import" href="bower_components/polymer/polymer.html">
        <link rel="import" href="an-ele.html">
        <style is="custom-style">
            an-ele {
                --an-ele-bgname: red;
            .name {
                color: yellow;

Full source: an-ele.html

<link rel="import" href="bower_components/polymer/polymer.html">

<dom-module id="an-ele">
        .name {
            display: block;
            background-color: var(--an-ele-bgname, pink);
        <span class="name">hi</span>
            is: "an-ele"
polymer polymer-styles

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