I can only assume that the theme is hidden because the constructor wanted it that way; it cannot be because providing a clue to the theme would be too difficult. Doing so took me about 30 seconds.
In the upper right corner, substitute SMEWS and CITIES with OMAHA and MIMIC. That would give you ACT at 13-Down and a clue for the theme.O M A H A
M I M I C
S E T
The question for me is why hiding a theme would be preferable to exposing one. Assuming a solver knows to look at long answers for commonality, themes are not terribly challenging to divine. Perhaps there's some Freudian motivation I cannot fathom at work in this.
In any event, the LAT has won. Covert themes will remain just our secret."Test one's metal"
for ASSAY was a good one as was "Common autograph site"
for PLASTERCAST, though much the same entry appeared a few days ago.