Flutter - Architectures for designer/developer iteration process flow

I've been trying to figure out a good Flutter process workflow for designer and developer interactions in order to iterate desktop/mobile/web application design. There isn't a very clear line of demarcation between the code and the design as there is in web dev. (for example, the developer would be focused on javascript and a little html, while the designer would be focused on the css and html.)

Some of this has been written about here: how developers and designers can collaborate using flutter but the mechanics have been left out. (also, it seems like the average flutter designer needs to be more tech-savvy with git skills than designers of yore. Is this your experience?)

While there does seem to be good control of color/fonts/styles with the AppTheme widget, other design aspects are more difficult to enact. For example, if you wanted to change all of the div's (in html) / Containers() (in flutter) from square corners to rounded, you could set a class and apply it across the board in web dev. In flutter, I believe you have to visit every Container in the code and apply a BoxDecoration widget... and repeat if there are changes in the future.

Specifically, what design-dev processes/architectures are you using to facilitate design iterations between devs and designers?

Note: Additionally, I've seen that there are some tools like Adobe XD, and supernova.io that allow for design exporting to flutter code, however that seems like a one-time thing... and doesn't lend itself to well to iteration. Has anyone used this method in their design iterations?