Opinon: New Pages in Old Theme?

I have a current custom WordPress theme, which is quite old now lots of unnecessary pages, functions and styles.

A new build and style is being introduced but only a page at a time. e.g new home page, then a new about page, whilst all the other pages run of the current old theme. Ideally i really i want a new theme completely for the new pages, but thats not practical as 90% of the rest of the time still use the old one.

Could this be done with a child theme, or would it be best to have conditionals to say all styles/functions for the rest of the site follow this, new pages follow this…?

You could create a custom page template for single posts with custom body class and only use it on new posts.

I need to limit everything, the currently styles are complete mess and so are the functions.

I think they only way is to set up conditional’s for a whole new function.php