It would be very helpful to allow TemplateStyles to be stored with Index pages at Wikisource. Currently, only .css pages in Template namespace get the sanitized-css model, which is a little bit awkward for CSS that really belongs to an Index page.
However, this might run into issues since subpages of Index pages get slapped with the edit form for the index, e.g. https://en.wikisource.org/w/index.php?title=Index:Sandbox.djvu/styles.css&action=edit
Somewhat related to T215165 and T226275 (which are about allowing index pages to specify the CSS to use for the pages of the index and have it applied automatically on transclusion too.