Picture entity: dark mode variation doesn't seem to work

I want to change an image (picture entity) when my iPhone goes into dark mode. I found that I should use

type: picture
image: /local/pictures/areas_text/woonkamer.png
dark_mode_image: /local/pictures/areas_text/woonkamer-dark.png

But it does not seem to work. Obviously the images are both in that location. It only shows the non-dark image, because I set that earlier. I added the dark-mode into the yaml later.
I also deleted the front-end cache, but no luck.

When I dug deeper, I saw I need perhaps themes. But is that true? I do not use themes and let the app follow my iPhone dark mode settings. How do I change the image accordingly?

Are you using picture card? I don’t see dark_mode_image listed in its documented options: https://www.home-assistant.io/dashboards/picture/

Nevermind, the picture element card is working…