title: Use custom properties in the CSS with Polymer
body: <p>I'm making a webapp using Polymer. I have a custom element that contains a paper-button. I have to set the ripple color of said button according to an attribute (or property?) so that in my main html file I can write<br />
  <code>&lt;my-element ripplecolor="red"&gt;&lt;/my-element&gt;</code><br />
  I tried something like<br />
  <code>&lt;paper-button style="--paper-button-ink-color: {{ripplecolor}};"&gt;&lt;/paper-button&gt;</code><br />
  but it doesnt work.</p>