GOV.UK Vue doesn't provide any of the Frontend styling directly, it's up to the consuming project to include it. I've done that so that you can add Vue to server-rendered projects which already include Frontend.

Frontend updates are often CSS-only, so those will be taken care of by the consuming project updating it. For markup changes and new components, that's a manual process on my side. Markup changes are usually minor and new components are rare, so it's not too much effort to keep up. I work on GOV.UK services in my day job so I've always got an eye on what's coming up.