What you could do is use WordPress as a REST API server, and use React as a client for that. Now you can have WordPress deliver your assets, but you could also put it in a subdirectory, and treat it only as an API app with a CMS admin area, while putting your site files in the root, by-passing configuring a theme at all. (SEO would become a separate concern here that you'd have to solve, depending on the type of app you are developing.)
Alternatively, you could use React.js to only 'enhance' the theme (like for a comments section), but you'd mostly be a PHP/WordPress dev at that point.
Isomorphic is like Meteor.js, and projects like that, that let you specifically avoid PHP (and avoid WordPress). Mutually exclusive really. If you are really interested in isomorphic, try Meteor.js and avoid using WordPress altogether.