Bug 56637 [Preferences] DialogMessageArea image is too large
Is the background color of the error shown by DialogMessageArea to be changed? Or do you intend to leave it as it is right now? The error message looks a little out of place in the current implementation (being surrounded by white space).
bundles/org.eclipse.jface/src/org/eclipse/jface/preference/PreferenceDialog.java
* is different than an error message (set using <code>setErrorMessage</code>). 
	 * An error message overrides the current message until the error message is 
	 * cleared. This method replaces the current message and does not affect the 
	 * error message.
	 * </p>
	 *
	 * @param newMessage the message, or <code>null</code> to clear
	 *   the message
	 * @param newType the message type
	 * @since 2.0
	 */
	public void setMessage(String newMessage, int newType) {
		messageArea.updateText(newMessage,newType);
	}
	/**
	 * Sets the minimum page size.
	 *
	 * @param minWidth the minimum page width
	 * @param minHeight the minimum page height
	 * @see #setMinimumPageSize(Point)
	 */
	public void setMinimumPageSize(int minWidth, int minHeight) {
		minimumPageSize.x = minWidth;
		minimumPageSize.y = minHeight;
	}
	/**
	 * Sets the minimum page size.
	 *
	 * @param size the page size encoded as
	 *   <code>new Point(width,height)</code>