styles.html
<style>
button {
background: red;
}
</style>
my-button.html
<link rel="import" href="../bower_components/polymer/polymer.html">
<link rel="import" href="../styles/main-styles.html">
<link rel="import" href="../behaviors/mixins.html">
<dom-module id="my-button">
<template>
<button>{{text}}</button>
</template>
</dom-module>
<script>
Polymer({
is: 'my-button',
behaviors: [ButtonBehavior]
})
</script>