I want to know how many times certain repeating themes are used by users.
You can simply search in top bar search. If you want more specific search like how many times an specific word is used by specific user then in search bar you can simply write
"desired phrase" user:userId.
You need to replace "desired phrase" with whatever word or sentence you want to search and "userId" with specific id of the user which you can get by visiting user profile.
As said by @Triyugi Narayan Mani in their answer, there is no need of a query of this. It can easily found by search from top bar.
Visit How do I search. There are many search operators with which we can find desired word or a phrase easily.
Anyways, I found a data query with which we can find a word or phrase written by a user in all of their posts. Search posts by a user. Just a note, SEDE updates once in a week.