How can I switch between the title menu themes?

Apparently, you are not able to do this manually. As you've noticed, the title screen changes once you have beaten the game.

From this thread:

Delete your save files, lol. :-) Hope this helps your problem! So far that is the only solution, I really hope they let us change it in the future.. Or cycle them randomly everytime you open the title screen.

And from, this thread, the desire to change titles at will implies the lack of ability to do so:

I personally wish you could choose the original title screen musice (love the Somnus theme more than the one we get after winning).

However, from the same thread, it looks like the background (but not the logo) changes on a day-night cycle:

User 1: ...but can you change it back to the original title screen? I kinda liked it.

User 2: i'm not really sure, maybe if you start a new game and then return to title it might switch back.

User 3: nahhh im running a second playthrough since a couple of days, and it did not change back. i rly hope there is some way to do it, because i rly prefer the darker one.

User 2: oh if you just meant the background and not the logo then it's a day night cycle that it rotates through.