CVI panels and UI objects are in a proprietary format that cannot be customized with style sheets or in other ways. The only possibility you have to modernize your UI (whatever that means) is to use built-in tools and features in a creative way.
Some people out there have suggested to try integrating CVI code with more up to date UI development environments but this is a path that you will have to follow with very few companions and even less documentation. Anyway, there are a few discussions you may want to search and read on this: this one for example can offer some interesting starting point.
