If it's a good idea, and we're going to discuss the proper level of the software stack to situate it, this should go in the UI toolkit for the desktop environment. (Why should the web browser be special?)

Alas, the state of the web today is that every website should feel the need to ignore my desktop settings and re-invent the UI — to some degree of success. I have a beautiful theme, and it seems to me a shame that web browsers (and sites) ignore it. We should not be re-inventing the UI for each web site we design. (And it should not be the websites implementing basic UI elements.)

Better, I think, would be a way to hook into the user's UI toolkit and UI preferences, and theme them. That way, the web site behaves like the rest of the UI environment, but can be themed to match your corporate style. (If you've ever seen FreeCiv on Linux, that might be a good example: it uses GTK, and the settings, but themes the widgets for the game.)