[Polymer] Custom Elements: Styling

Code: 

<dom-module id="business-card">
    <template>
        <div class="card">
            <h1>Joe Maddalone</h1>
            <h2>Instructor</h2>
            <h3>egghead.io</h3>
        </div>
        <style>
            .card{
                background-color: #e8e8e8;
                box-shadow: 0 0 1px #e8e8e8;
                position: relative;
                font-family: monospace;
                display: flex;
                width: 350px;
                height: 200px;
                flex-flow: column wrap;
                margin: 20px;
            }
            .card:before, .card:after {
                z-index: -1;
                position: absolute;
                content: "";
                bottom: 15px;
                left: 10px;
                width: 50%;
                top: 80%;
                max-width:300px;
                background: rgba(0, 0, 0, 0.7);
                box-shadow: 0 15px 10px rgba(0, 0, 0, 0.7);
                transform: rotate(-3deg);
            }
            .card:after {
                transform: rotate(3deg);
                right: 10px;
                left: auto;
            }
            .card h1,.card h2,.card h3 {
                font-weight: normal;
                color: var(--custom-text-color,--text-color)
            }
            .card h1 {
                flex:4;
                padding-top: 50px;
                font-size: 24px;
                align-self: center;
            }
            .card h2 {
                flex:3;
                margin-top: -30px;
                align-self: center;
                font-size: 12px;
            }
            .card h3 {
                flex:1;
                font-size: 14px;
                align-self: flex-end;
                margin-right: 20px;
            }
        </style>
    </template>
    <script>
        Polymer( {
            is: "business-card"
        } )
    </script>
</dom-module>

Using parent compoment in css :host

• remove the <div class="host"></div>
• change .card class to :host, which can be consider as <div class=":host"><h1>..</h1><h2>..</h2><h3>..</h3></div>

<dom-module id="business-card">
    <template>
        <h1>Joe Maddalone</h1>
        <h2>Instructor</h2>
        <h3>egghead.io</h3>
        <style>
            :host {
                background-color: #e8e8e8;
                box-shadow: 0 0 1px #e8e8e8;
                position: relative;
                font-family: monospace;
                display: flex;
                width: 350px;
                height: 200px;
                flex-flow: column wrap;
                margin: 20px;
            }

            :host:before, :host:after {
                z-index: -1;
                position: absolute;
                content: "";
                bottom: 15px;
                left: 10px;
                width: 50%;
                top: 80%;
                max-width: 300px;
                background: rgba(0, 0, 0, 0.7);
                box-shadow: 0 15px 10px rgba(0, 0, 0, 0.7);
                transform: rotate(-3deg);
            }

            :host:after {
                transform: rotate(3deg);
                right: 10px;
                left: auto;
            }

            h1, h2, h3 {
                font-weight: normal;
                color: var(--custom-text-color, --text-color)
            }

            h1 {
                flex: 4;
                padding-top: 50px;
                font-size: 24px;
                align-self: center;
            }

            h2 {
                flex: 3;
                margin-top: -30px;
                align-self: center;
                font-size: 12px;
            }

            h3 {
                flex: 1;
                font-size: 14px;
                align-self: flex-end;
                margin-right: 20px;
            }
        </style>
    </template>
    <script>
        Polymer( {
            is: "business-card"
        } )
    </script>
</dom-module>

Create variable in css:

:host{
                --card-color: #e8e8e8;
                --text-color: #222;
            }
            :host {
                background-color: var(--card-color);
                box-shadow: 0 0 1px var(--card-color);
                ...
            }    

Default value:

                background-color: var(--custom-card-color, --card-color);
                box-shadow: 0 0 1px var(--custom-card-color, --card-color);

--card-color will be the default value if --custom-card-color not exists.

------------------------------

index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Polymer</title>
    <script src="bower_components/webcomponentsjs/webcomponents-lite.js"></script>
    <link rel="import" href="bower_components/polymer/polymer.html">
    <link rel="import" href="my-card.html">
</head>
<body>
<my-card></my-card>
</body>
</html>

my-card.html:

<link rel="import" href="./business-card.html">
<dom-module id="my-card">
    <template>
        <business-card></business-card>
        <business-card class="red"></business-card>
        <style>
            .red{
                --custom-card-color: red;
                --custom-text-color: white;
            }
        </style>
    </template>
    <script>
        Polymer( {
            is: "my-card"
        } )
    </script>
</dom-module>

busniess-card.html:

<dom-module id="business-card">
    <template>
        <h1>Joe Maddalone</h1>
        <h2>Instructor</h2>
        <h3>egghead.io</h3>
        <style>
            :host{
                --card-color: #e8e8e8;
                --text-color: #222;
            }
            :host {
                background-color: var(--custom-card-color, --card-color);
                box-shadow: 0 0 1px var(--custom-card-color, --card-color);
                position: relative;
                font-family: monospace;
                display: flex;
                width: 350px;
                height: 200px;
                flex-flow: column wrap;
                margin: 20px;
            }

            :host:before, :host:after {
                z-index: -1;
                position: absolute;
                content: "";
                bottom: 15px;
                left: 10px;
                width: 50%;
                top: 80%;
                max-width: 300px;
                background: rgba(0, 0, 0, 0.7);
                box-shadow: 0 15px 10px rgba(0, 0, 0, 0.7);
                transform: rotate(-3deg);
            }

            :host:after {
                transform: rotate(3deg);
                right: 10px;
                left: auto;
            }

            h1, h2, h3 {
                font-weight: normal;
                color: var(--custom-text-color, --text-color)
            }

            h1 {
                flex: 4;
                padding-top: 50px;
                font-size: 24px;
                align-self: center;
            }

            h2 {
                flex: 3;
                margin-top: -30px;
                align-self: center;
                font-size: 12px;
            }

            h3 {
                flex: 1;
                font-size: 14px;
                align-self: flex-end;
                margin-right: 20px;
            }
        </style>
    </template>
    <script>
        Polymer( {
            is: "business-card"
        } )
    </script>
</dom-module>