I was lucky to find someone who can both design well and implement the changes in the codebase (using Tailwind CSS which I'd highly recommend). You can obviously have two separate people (one to design, one to implement), but if you can find someone who can do both it will simplify things.

For Preceden, we kind of just went page by page converting the old design over to Tailwind, making improvements along the way, and gradually ripping out all of the old CSS that had accumulated over a decade of me working on it. As I built out new features he would help with the design, but he spent the majority of his time improving the existing design.