I assume you’re suggesting loading and unloading themes as needed? There’s nothing wrong with that, in fact my approach will grow your app.scss file pretty substantially as your themed things grow and the lazy loading of assets could become a factor.

However I think my approach has a strength in that you only need one line of code to theme a CSS attribute for a given element. If you’re maintaining the same theme style across style sheets, I think you’ll quickly find that becomes hard to maintain and hard to keep things in sync.