I presume this is a browser/cache issue rather than a xenforo bug.
go to new posts
restore dismissed notices.
dismiss the notice (NB: I have only tried with floating)
click on a post
click on browser back button
I am finding with Firefox & Chrome, sometimes the dismissed notice comes...
Inspecting the elements, when I right click and choose hover on the CSS, there's no background property. If I try adding one, even with !important, it doesn't appear to work:
Edit: Looking at it again, it looks like it put dots on the four corners, but that's not what I want.