This shadow contains CSS to customise the TiddlyWiki and is left empty in the [[vanilla]] TiddlyWiki, so you can add any customisations. It's applied last, after applying the other StyleSheet shadows.

Generally speaking, you should only override this shadow and not any of the StyleSheet<something> shadows, as those are subject to change in future editions of TiddlyWiki, so changing those would require you to merge your changes when upgrading.