jQuery Mobile comes from the same community that builds jQuery Core & UI. So ultimately one should expect to have all the UI Components available with the Web Framework on Mobile.
The only downside I can think of right now - is the inability to “progressively” integrate jQuery Mobile with existing Web Pages.
By this it means that if you want your existing Web Page, that doesn’t obey the jQuery Mobile HTML Semantics to use a jQuery Mobile UI Element like a Slider – then it wouldn’t work. I see this as a drawback because all the existing Web Pages must be rewritten to obey the framework Semantics, which is a pain
jQuery Mobile follows the Theming framework which works well with jQuery UI for customization. So, the answer is Yes – customization is possible with Theming