White text on a black background. Yellow on grey. Puce on green. Not only are they unreadable -- for those who spend most of their screen time looking at things with a white background, these inverted sites mess up the eyes' adaptation. Read a couple of paragraphs of white on black and, when you go back to a more conventionally formatted page, the afterimage will degrade the reading experience for half a minute or more.
I use the bookmarklet below every time I encounter an inverted page. It changes the background to white and the type to black. As my eyes age, I'm finding the greater contrast helpful even on those modern, design-heavy sites that use grey-on-grey.
I found this bookmarklet years ago on some hacker's Web site; no idea who or where, and I can't find any trace of it online now. So I'm posting it here as a public service so others may find it.
Drag the link to your bookmarks bar and when prompted, give it a name. I have named mine "mono."