How to refine top-right search bar results?

Hi there!

How can I refine the top-right search bar results to be slightly more inclusive?

For example, when I search “come thou” there are no results. But when I search “come, thou” there are three.

I thought I would be able to use a plugin like Relevanssi, but it appears not to affect that particular search bar, only the search bar block that I can manually insert to pages/posts.

Any ideas would be much appreciated.

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