ProofreadPage Index: pages currently allows to inject some CSS to Page: pages. It would be nice to have a proper system based on TemplateStyles that allows to insert a stylesheet on Page: pages and main namespace pages that transcludes the pages from the same Index:.
Idea from: https://en.wikisource.org/wiki/Wikisource:Scriptorium/Help#Template_styles_to_reduce_calls_to_ts...
See also T200632: Allow template parameters to provide CSS to a templatestyles stylesheet