Theme?

[Gizzit]

I've got today's Guardian crossword half-completed, but the instructions say:

"Sixteen across clues and 15 down clues lead to the heart of their solutions. Their required entries in the grid all display a common feature".

I cannot see the theme...can anyone enlighten me?

[JJ109]

I've just finished!
There is a definition and then the parsing only gives the "middle" of the definition. The first two letters and the last two letters are unclued but are the same. Therefore if the first two letters are AZ for example, the last two letters are also AZ

[JJ109]

I also think there are 2 mistakes
2d has no definition in error I believe
26d looks like it is thematic, but it parses fully. Therefore I only have 14 down clues that are thematic!

[JJ109]

^^ sorry 36d

[Gizzit]

Thanks for that - takes me down to 12 clues left to solve.