Rebranding application [closed]

I'm trying to figure out the best way to rebrand my application. It's a electron application, thus build on react.js The company that i work with, sells this application as a SaaS solution, but we are now looking to sell the application to another company, with us being the maintainer of their version of the application. The application isn't going to change all that much, but some parts need to be altered. I'm talking API URL's, logos, company name, etc.

Now the best solution that i could come up with, fork the repository, and pull any new code into they version of it.

Pros:

• Versioning will be easier to maintain, and any changes made for the customer, can be pulled into our application.
• No clutter of "if(isThisCompany) {useThisLogo()} else {useThatLogo()}"
• SLA might dictate that any new code isn't going to be implemented in their version of the application. This is easier to control.

Cons:

• Updating our version of the application doesn't automatically update their version. It needs to manually merged into their application.

My colleague hates the idea, and says it will be the devil to maintain. Especially if more customers was to buy the solution in the future. I don't agree with him.

Do you guys know of any great way to solve this? Maybe through some programming pattern, or via great packages?

I was looking into using environmental variables, but i cannot find a smooth way to implement the variables in the package.json file, which holds some key values, like the icon path of the builded macos version.