It would be nice to have a themeing mechanism baked into the browsers for custom elements, pages, and the whole app. Most UI Widget frameworks today have themeing built in, such as Jquery UI, KendoUI, Wijmo, ect ect. Oses and some other non-web applications have themeing, heck even our browsers can have custom themes. I know the Polymer team is working on making a solution for Polymer by creating custom, re-usable, sharable themes, but this is on a framework by framework basis.
I would like to see a spec for themes either for css as a whole or web-components. Another source of inspiration is Asp.net themeing, you can theme a whole app, individual pages, partial pages, or individual components. A standard mechanism would be very useful now that we have custom elements as a standard, almost :), but themes should not just be limited to custom elements they should appy to our existing built in ones as well, buttons, selects, ect. Thanks, Dan