How is work distributed between the designer, backend and frontend programmers when finalizing the interface?

Question

Yii

There is a project implemented on Yii2 - personal user accounts with a large number of tables, graphs, forms. Now the interface is standard from Yii2, there is a need for a significant improvement in the design of the interface: not only minor CSS changes, but a significant refinement of the HTML widget templates.
The programmers who are currently working on the project have good experience only in the backend. What is the best way to organize the interface redesign work?

Answer 1

There are two options:

• We leave boostrap as the basis, then 70% of the widgets will not have to be edited.
  
• Drawing the layout, typesetting, redefining widgets.
  

Answer 2

Unfortunately, it will be difficult to do something, for the future, look towards the Single Page Application, there is a very clear separation, API + design (layout).