Download a theme zip file, unzip it, then
add the following line inside your <head> tag, after all other HTML imports:
<link rel="import" href="path/to/theme.html">
To use the theme within your custom element, add the following line inside your
<dom-module> tag:
<link rel="import" href="path/to/theme.css" type="css">
Alternatively, you can download all the free themes using bower:
bower install polymer-themes
Click the Polymer 0.5 button for a theme to download the CSS file, then
add the following line inside your <head> tag:
<link rel="stylesheet" href="path/to/0.5/theme.css" shim-shadowdom>