I don’t think there is a NEED to find something else from what is already understood by people generally due to the use of this term in almost every forum software and beyond.
That said, if we are really looking for terms to describe smaller composable blocks for themes then we can create a new term. Ideally, we should not do that, but using any existing term would cause confusion with various other usage in practice. The final composed outcome as presented to the user should still be called themes.
One such term that I can make would be THEMELETS which can be thought as small themes or a word-play of theme + template (= theme templates).
After writing this, I thought I would Google this term and I found that some people have already used it (check the following three examples).
https://gist.github.com/Robert-Frampton/27293bcee119f4ab5fb4