I suspect it's more likely that one of the changes made while migrating the site to the new server has flushed this out. However there aren't any obvious references to those files using http://, which means that wherever it's happening, it's probably a subtle side effect of one of the rewrite rules in place.
The page code in main.html I see is quite explicit:
<link href="/default2.css?v=6155" rel="stylesheet" type="text/css">
<link href="/plaza2.css" rel="stylesheet" type="text/css">
So the only way that loads using http:// is if the original page request was http:// too, which is odd, because if you try to load
http://www.grometsplaza.net/main.html the server should redirect you to the https:// version before it ever gets to loading the CSS files. If it's reproducible for you, a screenshot of the Network tab with the main.html request selected might help narrow it down.
Is anyone else seeing this?