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
/**
	 * Layout for the page container.
	 *
	 */
	private class PageLayout extends Layout {
		public Point computeSize(Composite composite, int wHint, int hHint, boolean force) {
			if (wHint != SWT.DEFAULT && hHint != SWT.DEFAULT)
				return new Point(wHint, hHint);
			int x = minimumPageSize.x;
			int y = minimumPageSize.y;
			Control[] children = composite.getChildren();
			for (int i = 0; i < children.length; i++) {
				Point size = children[i].computeSize(SWT.DEFAULT, SWT.DEFAULT, force);
				x = Math.max(x, size.x);
				y = Math.max(y, size.y);
			}
			if (wHint != SWT.DEFAULT)
				x = wHint;