I was under the impression that a select element could be styled in the same way as an input element using 'mw-ui-input' [1] as essentially they are one field.
This came up when I tried to enable MediaWiki UI on select elements in all forms.
If you are curious what this would look like it would look like this: http://tools.wmflabs.org/styleguide/desktop/select.html
Is my impression true? If yes, we should update the documentation to explicitly say that. If no, we should update the documentation and describe how they should be styled...
[1] http://tools.wmflabs.org/styleguide/desktop/section-1.html