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PolymerJS: Allow custom elements to be styled: many rules with mixins

Previously, we showed how one CSS property in your custom element could be styled from the outside using a shim for CSS variables.

But we often want to allow the user to style not just one rule for a class, not even many, but anything for that class.

Polymer 1.0 uses an experimental feature called mixins that allows the element user to specify any CSS property for a class.

We @apply(--an-ele-name-theme) in CSS selector for the class in the custom element:

<dom-module id="an-ele"><style>
        .name {
            @apply(--an-ele-name-theme);
        }
    </style>
...

Then in the index.html, or hosting custom element, we specify the --an-ele-name-theme under the selector for our element, and add the styles there.

<style is="custom-style">
    an-ele {
        --an-ele-name-theme: {
            display: block;
            color: white;
            font-weight: bold;
            background-color: blue;
        };
    }
</style>

Notes:

is="custom-style" is only needed if your styling the element from index.html not a polymer element.

Full source: an-ele.html

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

<dom-module id="an-ele">

    <style>
        .name {
            @apply(--an-ele-name-theme);
        }
    </style>
    <template>  
        <span class="name">hi</span>
    </template>
    <script>
        Polymer({
            is: "an-ele"
        });
    </script>   

</dom-module>

Full source: index.html

<html>
    <head>
        <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-name-theme: {
                    display: block;
                    color: white;
                    font-weight: bold;
                    background-color: blue;
                };
            }
        </style>
    </head>
    <body>
        <an-ele>
        </an-ele>
    </body>
</html>

polymer polymer-styles

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