I get titles from a lot of places. Phrases (if too common, I try to twist them a bit), song titles or lyrics, something from the story itself if I know enough to come up with something that way (for my WIP, I'm using a locale I created on Mars as the title), or sometimes, something I see or overhear. I once borrowed a quote I saw on a statue and chopped off the first few words to make the title. For a story in my self-published spy series that delved into the psyche of the main character, I used a psych term. So sometimes, I guess I use the theme in the title, though I don't set out to write themes and in the cases where they do come clear to me isn't usually until I've written the story.
I used to jot down title ideas but never used them so stopped doing it.
Writing collaboratively (a topic I plan to discuss at some later point) makes coming up with titles a bit harder. For the on-hold Mars book, my co-author had an idea in mind, but couldn't find the words she wanted. The universe is her creation -- I just added to it -- so I had no problem with using her idea. So, when I was visiting her one day, we went through a thesaurus, looking up words that fit her concept until we found a two-word phrase that suited us both. It's not what I would have named the thing, but it's what has now become cemented in my mind.
And if a publisher ever asks for me to change a title? If I couldn't talk them out of it, sure, I'd change it to get the book published (same as for character names), but in my mind and heart, I would always think of it with the original name. After all, that's how I've identified them for a very long time.