Co-Owners to a WA world aren't able to see what "Sheets & Blocks" are in the Studio area.
When editing CSS within a "Sheets & Block" page the CSS won't effect the code in an article.
Our article is currently edited using the World Styling Editor, not the CSS in our Sheets & Blocks editors.
https://www.worldanvil.com/w/elaris-snakekeeper7/a/5e-feats-article
Hey The_Reaper_Angel, the Sheets & Blocks Studio is an account-based guild feature so co-owners to a world don't have access to it - this is intended. However, block templates and statblocks created from those in the Studio can be seen and edited within the world. As for CSS not affecting the article, make sure that the CSS you add for is either in the Presentation CSS editor (you can find it by clicking View World and select the palette icon on the right) for CSS styles that you want to affect all articles in the world. You can set CSS exclusively for a selected article in the Styling tab when editing an article.
Let me know if this information solves your questions, but from what you've reported the issue you're encountering isn't a bug.