I know only one extension that Opera will offer you to install it. That is, when you visit CWS -chrome web store, opera will offer you to install one of the in-house extensions, to help install other Chrome extensions into Opera.
And that is, only first time when you visit CWS... so, it doesn't repeat it self.
like @blackbird71 wrote, many sites can fake message styles, so that they look like Opera native bar (gray horizontal bar on top of the page). They may look like part of the Opera UI (or any other browser), but they not...they are part of the web page.
I know that websites like Amazon offering visitors to install extensions (this one is scary, it even added toolbar button instantly without me allowing installation)