Hey Julius,
Problem is, there’s no wpp.css under Styles in Editor, only style.css and custom.css. Am I missing something?
That’s because they had previously copied the wpp.css stylesheet (wordpress-popular-posts/style) into their theme’s directory which is why they were able to access it via wp-admin > Appeareance > Editor.
Since you haven’t done that, you can only find it via Plugins > Editor > WordPress Popular Posts.
After you’re done making changes to the stylesheet, you should copy it into your theme’s directory. Otherwise, next time you update WPP all changes will be lost since all files are overwritten automatically by WordPress.