IMO, a single monolithic mother stylesheet goes against all that TW stands for and we should go in the exact opposite direction; We should split up the current theme(?) stylesheets into meaningful chunks - BUT then use the usual filter techniques to assemble them into some kind of overviewable aggregation that also sets the cascading order of them.
(Crazy idea: Maybe tiddlers could themselves be style definitions? The title is the selector and the text is the declarations. Maybe fields for sub selectors. This way it'd be very simple to reuse declarations. And it would be super simple to try out stuff; just overwrite such shadow style tids and delete if you regret. And we could tag styles or add other meta data.)
IMO, CSS hackability is one of the weaker areas in TW if you consider that hackability is a main objective and how powerful CSS is in TW.
<:-)