How to get background color information for an element using Poltergeist and Capybara

I'm trying to move from testing with Firefox and Chrome to Poltergeist for integration tests. One thing I am struggling with is getting the background color of an element.

Using Capybara / Selenium this works: find (: css, "locator"). native.css_value ('background-color')

Using Capybara / Poltergeist I am getting an error that css_value is an undefined method.

Any suggestions would be appreciated.