No, they will not.

Googlebot will use the resources your site depends on (CSS, JS) to render your pages, but all that Google indexes are web pages.

Google understands web pages by their HTML markup, images, and structured data.

It would not be very reliable for Google to pay attention to, for example, WordPress themes due to the vast number of sites that use the same theme.

No, they will not.

Googlebot will use the resources your site depends on (CSS, JS) to render your pages, but it will not index them. These files are not of any use for ranking. Web pages are understood by their HTML markup, images, structured data, and other pages that link to them.

It would not be very reliable for Google to pay attention to, for example, WordPress themes due to the vast number of sites that use the same theme.

What types of files may get indexed?

99% of what Google indexes are web pages and images. Occasionally, you might stumble upon an indexed sitemap.xml or robots.txt, but you'd have to really be looking for those to find them (eg. site:example.com/sitemap.xml). While it is unlikely that a normal searcher will ever come across them, you can safely serve a X-Robots-Tag: noindex header to prevent this from happening.