This is the new, standart way to define variables in CSS. Currently it's only supported by Firefox (v31+), but Chrome also expressed intent to support. You can also see the usage of CSS custom properties throughout Polymer via their custom style element.

For further info you can check these out: (1): http://dev.w3.org/csswg/css-variables/ (2): https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables (3): https://www.polymer-project.org/1.0/docs/devguide/styling.html