Why is the site background not showing on some devices?

Question

css

Good afternoon.
There is a site where there is a button to change the theme (I'm trying js). When you change the theme to light, the background changes, the background is in the form of a picture in png format (the usual pattern is a pattern). I'm ok. The background is displayed. There is another PC nearby, the same Win 10 operating system, the same browsers - but there is no background. If on the 2nd try to pull out the picture, then in the end there will be only a white background (in Photoshop, you can reduce the brightness and the pattern becomes visible). On the phone, you can just see the borders of the pattern, but the picture itself is not visible.
I sent a link to a friend, everything is OK with him, like me. What can be wrong?
The site itself: screbok.net23.net
Ps I tried to replace the image with jpg format, to no avail.