Wallpaper stretched vertically on mobile

So I use a background on the front end that is configured via the theme I use.
I use this line in my theme.yaml to accomplish this:

lovelace-background: 'center / cover no-repeat url("/local/images/dark.png") fixed'

The problem is that when i use the iOS app or the browser on my mobile phone the wallpaper is stretched vertically, when the amount of cards does not fit on the screen and i need to scroll, if that makes sense. If all the cards fit on the screen, the wallpaper fits nicely and looks good.
The stretched wallpaper looks like this:


The correct placement of the wallpaper looks like this:

As you can see on the first screenshot, the wallpaper looks stretched all the way down, where the second screenshot has a nice fit.
I hope someone can help me accomplish the same wallpaper fit for screenshot 1, so the wallpaper is fixed and does not stretch.

Thank you in advance!