Define a paint constant in your CSS file as follows:

.scene {
    palConstantFill: rgba(204,204,204,1.0);
}

Then reference that color somewhere else in your CSS file (for a background color or something), or reference it dynamically at runtime in your code via setStyle and receive the following exception/warning:

WARNING: com.sun.javafx.css.StyleHelper lookup caught:
java.lang.ClassCastException: java.lang.String cannot be cast to javafx.scene.paint.Paint
at com.sun.javafx.scene.layout.region.StrokeBorder$BorderPaintConverter.convert(StrokeBorder.java:522)
at com.sun.javafx.scene.layout.region.StrokeBorder$LayeredBorderPaintConverter.convert(StrokeBorder.java:496)
at com.sun.javafx.scene.layout.region.StrokeBorder$LayeredBorderPaintConverter.convert(StrokeBorder.java:481)
at com.sun.javafx.css.Value.convert(Value.java:243)
at com.sun.javafx.css.StyleHelper.lookup(StyleHelper.java:934)
at com.sun.javafx.css.StyleHelper.lookup(StyleHelper.java:868)
at com.sun.javafx.css.StyleHelper.transitionToState(StyleHelper.java:670)
.....
WARNING: com.sun.javafx.css.StyleHelper lookup styleable = 'artificial' StyleableProperty {property: -fx-border-color, converter: LayeredBorderPaintType, initialValue: null}
WARNING: com.sun.javafx.css.StyleHelper lookup resolved = <Value>
  <value values="1">
    <Value>
      <value values="4">
        <Value lookup="true">
          <value>palConstantFill</value>
          <converter>null</converter>
        </Value>
        <Value lookup="true">
          <value>palConstantFill</value>
          <converter>null</converter>
        </Value>
        <Value lookup="true">
          <value>palConstantFill</value>
          <converter>null</converter>
        </Value>
        <Value lookup="true">
          <value>palConstantFill</value>
          <converter>null</converter>
        </Value>
      </value>
      <converter>BorderPaintType</converter>
    </Value>
  </value>
  <converter>LayeredBorderPaintType</converter>
</Value>

I hope that there's a simple fix on my end for this, otherwise it's going to be tough to evaluate b40.