Sorry but I am new to using Fiddler.. so bear with me..
I have checked that the static files are accessible from browser, and but giving direct URL the files are correctly loaded.
But when I load the site, only the text, some basic controls (textboxes etc.) and content are loaded, as well as static images.
I can also see the files loaded through Fiddler.
What I cannot see is any request or fetching of the CSS file.. that is for me a bit strange.
The Site structure is basic and is according to the DNN default site.
So the portal is 0, it is using a host skin located in the _default portal folder, under /portals/_default/skin/gravity/
where also the skin CSS is located.
Just to be safe, I also have the same skin installed on the site, in case there would be an issue.
I am not seeing that the site loads ANY skin elements, or skin files at all.
so I checked the permissions on the _default folder and the sub folders.. any they are correct, and have not changed since before the upgrade (when the site was working, and the skin was fine).
If you want/or care to spend a few minutes checking the site it would help, and I would be grateful.
I will mail you the URL.