Tumblr themes still work like this. Tumblr is actually an almost-perfect natural experiment on the value of customization, because there are two views of any given blog post: a public-indexed static HTML view, that the post's author can theme with whatever HTML+CSS+JS they desire; and an RSS-reader-like "dashboard" view, with no theming at all.

Polling Tumblr users on how they prefer to read pages—whether in the dashboard web-app, or by opening the links to the standalone HTML versions—would be pretty solid data on whether customization makes content overall better or worse from consumers' perspective.