sql query

Hi what sql query will I use to checks the "users_theme" table and finds all the user_id's which exist in this table and not the "users" table. Once I have those id's that exist in the user_theme table that does not exisit in the users table I want to delete all rows with that user id from the users_theme.

for example In simple terms if user_id: 2546 exist in "users_theme" table but does not exist in "users" table delete the row of 2546