Roger may have a far superior approach for you, but here should be one way to apply a style to the login page based on the previous default page.
Most if not all GSP pages inherit from /CodeFiles/GalleryPage.cs, so you could add the stylesheet in a GalleryPage.cs method like AddUserControls() where we can see that the page is the login page (PageId == PageId.login) and then add a CSS based on the Request.UrlReferrer.
It requires some coding, and I don't have access to GSP in Visual Studio so I'm winging it, but it should do the job.
Good luck,
Dave
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