Separating content and style does not really have a purpose in GUIs. A lot of people who come from HTML assume it does because HTML does it, but HTML does it because it's for documents, where it does have a purpose. It's got nothing to do with declarativeness; everyone does the same thing because it works and the other way doesn't solve problems.

The only non-DOM-based library I know of to use separate stylesheets is JavaFX and it's one of the biggest reasons people hate it.