One functionality, several design and layout options - how to implement it?

Question

JavaScript

The situation is as follows: there is one site with certain functionality - registration, personal account data, etc.
However, there is a requirement to make many clones of this site, but with a different design and layout.
Interested in the question of how to do it technically without duplication. It is clear that in theory only css files can be changed, but, alas, this is not the case here - both layout and css classes change.
Now the site is written in yii and, in principle, you can make a lot of themes (yii themes), but this means that you will have to duplicate the layout, forms, write another js (however, the latter, most likely, will have to be done anyway).
The question is: can there be another technical approach to solving this problem today? Make a REST JSON API for example or use node.js

Answer 1

what about cms magento? there is such a bun

Answer 2

well, clone the folder with views and under each folder your own media folder with css, js and images, depending on the theme, load the desired folder with views

Answer 3

yii is not at all clear why here, but yes, take out all the management in a single admin panel, there is also one database, depending on the router (which domain came) you connect the template from the desired folder. Everything is ready.
Yes, layout, forms and other js will be required.
See how wordpess multisite is made for example.