I've been doing this recently - working with Qt on a local app.

I've had good luck providing a png "design board" with all of the template colors and having the first task be to build out a design gallery with all of the ui widget. Then have the design docs specify which component to use. Ensure that the documents specify to only use pre-existing components and have a list of each component and their intended use cases.

Of course, this learning came after seeing how awful V1 of the app was. Initially, it looked really impressive, but once you started clicking around it became obvious how incoherent the design was.

Claude's new frontend-design plugin is solid for web apps in my testing. My wife and I have been using it to build her an app and her discerning design eye is largely impressed with what it's done.