Making additional themes be in-app purchases seems like a reasonable way of funding the project. You might want to specifically say in the app description in the store what the in-app purchases are. I was kind of suspicious until I saw this explanation.
"Free on F-Droid, $3 on the Play Store" is another approach I've seen (e.g., Conversations and DAV5x).
"Free on F-Droid, $3 on the Play Store" is another approach I've seen (e.g., Conversations and DAV5x).