Search: add ability to customize the filtering widget's styles

This request comes from a user to be able to customize the styling on the front end for the search widget. Currently all of the styling is just inherited from whatever the theme is doing and we tried to do a good job selecting reasonably generic elements to add to the page. However, there are likely many themes where it doesn't quite turn out right, or cases where the user wants to be able to tweak the styling just a bit.

The issue names only the front-end search widget and its inherited theme styling; it does not identify a file, test, or entry point. Start by locating the search widget implementation and its existing styling, then determine how users should customize it. Done means the widget's styles can be adjusted without depending entirely on the active theme.

Written by the indexing model from the issue text.