The link to theme/images/no_avatar.gif is hardcoded into notification_dropdown.html, ucp_avatar_options.html and ucp_notifications.html. Not having the file in the current style results to a degraded user experience ("broken image" picture in the latest Chrome).

Therefore a style that inherits from prosilver has to either to have file theme/images/no_avatar.gif, or to overrride each of these templates. Neither is very good.

I suggest removing the hard-coded link from mentioned templates, and replacing it with a div with a css background-image pointing to this no_avatar.gif. This way an author of a new style can get by with some css-fixes only.

Or at very least document this as a mandatory element https://www.phpbb.com/styles/create/#a-mandatory