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How to style internal elements of a custom Polymer element using external styles?

It is not possible to style an element using the Shadow DOM with Polymer 1.x or 2.x. Consider the following custom item in Polymer 2.0:

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

<!--
`semantic-ui-button`


@demo demo/index.html
-->

<dom-module id="polymer-button">
  <template>
    <div class$="button {{size}}">{{label}}</div>
  </template>

  <script>
    class MyElement extends Polymer.Element {
      static get is() {
        return 'polymer-button';
      }
      static get properties() {
        return {
          label: {
            type: String,
            value: 'polymer-element'
          },
          size: { type: String }
        };
      }
    }

    window.customElements.define(MyElement.is, MyElement);
  </script>
</dom-module>

in the demo:

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, minimum-scale=1, initial-scale=1, user-scalable=yes">

    <title>polymer-element demo</title>

    <script src="../../webcomponentsjs/webcomponents-lite.js"></script>

    <link rel="import" href="../../iron-demo-helpers/demo-pages-shared-styles.html">
    <link rel="import" href="../../iron-demo-helpers/demo-snippet.html">
    <link rel="import" href="../polymer-element.html">

    <style is="custom-style" include="demo-pages-shared-styles"></style>
    <style>
      body { font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif; }
      .button {
        background: #ccc;
        border-radius: 4px;
        color: #444;
      }
      .button.big {
        font-size: 1rem;
        padding: 6px;
      }
    </style>
  </head>
  <body>
    <div class="vertical-section-container centered">
      <h3>Basic polymer-element demo</h3>
      <demo-snippet>
        <template>
          <polymer-button label="Demo"></polymer-button>
        </template>
      </demo-snippet>
    </div>
  </body>
</html>

The styles defined in the demo for .buttonand .button.bigdo not apply to the shadow element; however, Polymer 1.x uses styles if we use ShadyDOM:

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

<!--
`polymer-button`


@demo demo/index.html
-->

<dom-module id="polymer-button">
  <template>
    <div class$="button {{size}}">{{label}}</div>
  </template>

  <script>
    Polymer({

      is: 'polymer-button',

      properties: {
        label: { type: String }
      },

    });
  </script>
</dom-module>

Is there any way to select / style these inner elements using outer styles ?

Below is a visual representation of what I said above, in order of appearance:

  • Polymer 1.x using Shadow DOM
  • Polymer 1.x using ShadyDOM
  • Polymer 2.x

difference

+4
2

, CSS/mixins.

  • <style> :

    <dom-module id="polymer-button">
  <template>
    <style>
      .button {
        @apply --my-button-mixin;
      }
      .button.big {
        @apply --my-button-big-mixin;
      }
    </style>
    ...
  </template>
</dom-module>

  • mixin :

    <dom-module id="x-foo">
  <template>
    <style>
      polymer-button {
        --my-button-mixin: {
          background: red;
          color: white;
        };
      }
    </style>
    <polymer-button label="Red button"></polymer-button>
  </template>
</dom-module>

    ... index.html:

    <body>
  <custom-style>
    <style>
      polymer-button {
        --my-button-mixin: {
          background: red;
          color: white;
        };
      }
    </style>
  </custom-style>
  <polymer-button label="Red button"></polymer-button>
</body>

codepen ( 1)

codepen (Polymer 2)

+6

<style> @import url <template>, :

<template>
    <style>
         @import url( external.css )
    </style>
    <div class$="button {{size}}">{{label}}</div>
</template>

CSS (: external.css) CSS:

.button {
    background: #ccc;
    border-radius: 4px;
    color: #444;
}
.button.big {
    font-size: 1rem;
    padding: 6px;
}
+5

Source: https://habr.com/ru/post/1672126/

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