Sorry, but the PS5 launched without themes.
Here's a short answer for your question: You can't right now. Themes are not available on yet. I spent over an hour scouring my PS5 settings when I had received my unit just because I thought I was missing something, but nope — themes aren't there.
When I spoke with Sony, I was only told that "themes aren't available at launch," indicating that they could possibly be added at a later date. Given the popularity of them and how many people have been asking for their return, I think it's only a matter of time.
It looks like the alternative to themes right now are the built-in backgrounds that games have. Depending on whichever game you highlight on the home screen, the background will change to an image from that game. Toggle over Bugsnax and you'll see Elizabert Megafig on Snakooth Island. Spider-Man: Miles Morales, of course, has an image of Miles Morales in his suit. Music from the game's soundtrack will also play.
Once Sony adds themes to PS5, we'll update this with a step-by-step process on how to change them.
Greatness awaits
PlayStation 5
Next-gen is finally here
You can right now, and it's set to be Sony's most powerful console with a super fast SSD, 3.5GHz CPU, and 10.28 TFLOP GPU. As good as it is, you won't find home screen themes at launch.
All-digital
PS5 Digital Edition
Leave physical games behind
The PS5 Digital Edition packs in every bit of greatness the standard PS5 does, and does it for just $399. It might not have a disc drive, but the future is heading towards a more digital age every day. You'll just have to make do without themes right now.