How would you call this kind of background/effect on the corners?
>https://danbooru.donmai.us/posts/6105075?q=hxd
>https://danbooru.donmai.us/posts/6421672?q=hxd
>https://danbooru.donmai.us/posts/9391957?q=hxd
>https://danbooru.donmai.us/posts/6357183?q=hxd