The default tooltip is the same within a given GUI theme. You can change its style by creating a theme (if not done already), go to Theme -> Add Class Items, select either
TooltipLabel in the selector and click
Add all. This will add a style for tooltips in your themes that you can edit. Then, I guess applying the theme on your UI nodes will do the trick.
If you want a completely custom tooltip for different GUI elements (if the theme system isn't flexible enough), you could create a scene for the tooltip, then write a script that you put on GUI elements, detect mouse-over, and shows your tooltip scene.