Question: I have a story board file as the launch screen. I have set image in button background Image. In that image, I have an image set for light and dark mode. but when I switch to dark mode, it always shows light mode image.

Answer: 
I'm not sure if you still need an answer to this, but I recently had the same issue.
For me, what fixed this was to uncheck the box 'Preserve Vector Data' within the image set properties. This means resizing won't work on the vector image, but you will see the image change with light/dark mode toggled within the storyboard.
Alternatively if I go back and check 'Preserve Vector Data' (back to the original state) the Dark image will display when the app is running if the simulator phone is in Dark mode.
Make sure you have enabled Appearance to Any, Dark. And check that you have added images for dark mode appearance.

[![enter image description here](https://i.stack.imgur.com/q73Uy.png)](https://i.stack.imgur.com/q73Uy.png)