While applying the Sydney Pro Child theme, I discovered that there are non-https links, including the link to athemes. This is also true of the parent theme. As a suggestion, why not update those old links to the current ones by doing a search and replace in all your themes? I believe all of the URIs are now https, including the link to the GNU public license page.
I added this to our list, thank you for helping to make Sydney Pro better
Kind Regards, Roman.