Me again...

So, now that I can see my PGP signature buttons, I'm bothered because they
don't look like other buttons. Indeed, gnus-article-button-face isn't used to
display them. And that applies to mime buttons, too.

Is it intended?

If not, I suggest a simple fix: insert

    (when gnus-article-button-face
      (gnus-overlay-put (gnus-make-overlay b e)
                        'face gnus-article-button-face))

in both button creation functions (taken from gnus-article-add-button).
Patch attached.