Bug 53881 [Colors] Color/Gradient registry should return default values.
There should be default values for when a user does .get() on a bad key.
bundles/org.eclipse.jface/src/org/eclipse/jface/resource/GradientRegistry.java
/**
	 * Adds (or replaces) a gradient to this gradient registry under the given 
	 * symbolic name.
	 * <p>
	 * A property change event is reported whenever the mapping from a symbolic
	 * name to a gradient changes. The source of the event is this registry; the
	 * property name is the symbolic gradient name.
	 * </p>
	 * 
	 * @param symbolicName the symbolic gradient name
	 * @param gradientData an <code>GradientData</code> object
	 * @param update - fire a gradient mapping changed if true. False if this
	 *            method is called from the get method as no setting has
	 *            changed.
	 */
	private void put(String symbolicName, GradientData gradientData, boolean update) {
		Assert.isNotNull(symbolicName);
		Assert.isNotNull(gradientData);
		GradientData existing = (GradientData) stringToGradientData.get(symbolicName);
		if (gradientData.equals(existing))
			return;
		Gradient oldGradient = (Gradient) stringToGradient.remove(symbolicName);
		stringToGradientData.put(symbolicName, gradientData);
		if (update)
			fireMappingChanged(symbolicName, existing, gradientData);
		if (oldGradient != null)
			staleGradients.add(oldGradient);
	}
}