Generally speaking, crossword theme entries are all across clues, symmetrically placed, something like this:

However, if you’re a boss, you can make it work that the theme clues are STILL symmetrical, but some across and some down. This means that they have to cross each other symmetrically, e.g:

This repo contains a util that, given a set of theme clues, identifies the sets of cluse that have symmetrical crossings.

(Context: often I'll have one or two themers that I'm very excited about, and then a whole bunch more that I'm equally okay with, which leaves me free to choose based on grid suitability. In a case like this, I might bias towards themers that I can cross.)