Unfortunately since the add-on was written, pages have become a lot more complicated and it is harder to keep up.

If the page has been messed up by the add-on the Original button will restore it.

The Manual button is for when you aren't using Automatic mode. It either blackens or whitens the text. But it may get it wrong since it requires estimating the background colour of the text. This has become much more difficult these days. The Blacken button is supposed to unconditionally blacken the text, and White unconditionally whiten the text. If it doesn't then there is a bug.