Another user has encountered a similar problem and was desperately wanted to change back to the default wallpaper.
Android 9 has an auto-detect feature that will, theoretically, optimize your UI with your theme, and wallpaper was considered an important part of your theme. So what happens is, that it will set the icons to black line art when it detects a dark tone wallpaper, and it will set the icons to white line art when it detects a bright tone wallpaper. This turns out quite a disaster to the icons at N8ii Notification Center (i.e., the pull-down quick menu) because icons with black line art are hardly visible with the bray background. I personally think this theme-based synchronization is illogical, why would anyone want to use the black icon on dark wallpapers? Unfortunately, there is nothing much we can do.
I have created several wallpapers to explore this theme-based synchronization feature, and I have attached those that will keep the icons in white line art so that it is more readable, and the default N8ii wallpaper is one of them. Please copy these files to the PICTURE folder under internal storage of N8ii, and then change the wallpaper again, the notification icons should change back to the original white line art, without needing a reboot. You can use your own preferred photo as wallpaper, but please crop and resize the photo to 720x1280 and save them as jpg, bmp or png file format.
Regarding the violin picture titled wallpaper, I have checked with our service engineering and found out that the violin photo was the default wallpaper of HiBy R8. HiBy accidentally dropped that photo to N8ii firmware and the system has identified that as a viable alternative wallpaper. We shall remove the photo in the next firmware.