GEB get background and text color

I tried the jQuery and javascript approach to get the background or text color but I always got an Exception below.

groovy.lang.MissingMethodException: No signature of method: geb.navigator.NonEmptyNavigator.css() is applicable for argument types: (java.lang.String) values: [background-color]

Heres the code

class BrandingPageSpec extends GebReportingSpec {
    def "Scenario: Selecting a pre-set colours should preview the changes."() {
        expect: "currently at the branding page"
            at BrandingPage

        and: "advance settings hide successfully"
            normalSectionHeader.css("background-color") == "#FFFFFF"

        and: "stays at branding page"
            at BrandingPage
    }
}

class BrandingPage extends Page {

    static at = {
        waitFor(wait: true) {js."document.readyState" == "complete"}
    }

    static content = {
        normalSectionHeader{$('#section-header')}
    }

    static url = "http://someDomain:8080/branding"
}

h1{
  background-color: #FFFFFF
}

<html>
  <body>
    <h1 id="section-header" > Hello Word </h1>
  </body>
</html>

Any help would be appreciated. Thanks

Solution

Replace:

normalSectionHeader.css("background-color") == "#FFFFFF"

For:

normalSectionHeader.jquery.css("background-color") == "#FFFFFF"

FYI: http://www.gebish.org/manual/current/javascript.html#jquery_integration