It all depends what you mean. Some people use the notion of a theme sentence to refer to exactly the same thing as the main idea or big idea of the passage or message (the exegetical idea or homiletical idea in Robinson terms). Others mean something entirely less specific.
The main idea, or big idea, or theme and thrust, or proposition, or whatever you want to call it, should express both what the passage is about (the subject of the passage), and what it says about that (the complement of the passage).
When we have only the theme without the thrust, that is, the subject without completing it, then we may have a theme sentence of sorts, but really it is something significantly diminished. The theme without specificity is perhaps a title (though probably a weak one), but it is not the succinct, pregnant, clear, focused distillation of the details in the passage that is a main idea sentence.
If you have a theme sentence that is a couple of words long, and may not even be a sentence (i.e.a title lacking a verb or completion), then you do not have a sermon ready to expand into the time available. You have a title. You have a start. But to have the single sentence summary of the whole passage that is worth it’s weight in gold, be sure to complete the sentence, complete the thought. You’re preaching about God’s love? Great. What is the passage saying specifically about that?