Hi, this theme has been aroud for quite some time now but I'm not the author of the artwork. I contributed the code but I was not the only author. I am one of the main stakeholder of the documentation team of Plone and I had the idea to merge this theme into the sphinx core when I saw the pyramid one being there. I didn't though about the legal part actually so I made my researches and now I realize that, as you guessed, the logo is a trade mark of the plone foundation. I will remove it and make the theme more brand indipendent. I will make another pull request soon. Thank you.