I removed the {{fixme}} in the styles section. Based on my testing locally and in prod (English Wikipedia), the docs mentioned are entirely correct. Even in production, in debug mode the CSS is loaded directly from the original static file(s). I also tested with uselang=he, and CSSJanus is not applied (there's no derivative file for debug mode that has been flipped).
This post was posted by Mattflaschen, but signed as Superm401.