Sorry if I'm being slow here - so the solutions to the thirty clues which LACK thematic definition ARE in fact 'themed' by virtue of being 30 of a set of 50 things found in Brewers?
The 30 out of 50 are nicknames of US States - a ridiculously obscure theme which nobody could hope to know all the answers to without recourse to Wikipedia.
I haven't yet worked out what 'thematic definition' the remaining 28 clues share, though.
don't think that the other clues have a theme - the way that i read the instructions is that those 30 clues are lacking definitions and that those definitions have a theme
Having completed this, I now realise that whilst there are indeed 30 nicknames of US states some of them relate to the same state, so in fact I think only 24 of the 50 fifty states are represented - making the instructions given to solvers even more misleading!