Hi Peter,
That's a good question but unfortunately it has a very easy answer: You can't!
WikiTree controls the display style of links, which is green and underlined. The "popup" that displays when you hover over any link is browser controlled - it displays the web address of the link so that you know, before clicking it, where you will be going when you do.
Although using underline for links is a very dated practice in web page design and there are much prettier ways of doing it, it does make a lot of sense for WikiTree to control how links are formatted. That way, the look and feel is unified across the entire website, which helps makes it intuitive for users.