Search results should display in the theme in which the form was submitted

Problem/Motivation

In the core Help Topics module, we have added a search form to admin/help, which as an administrative page uses the default admin theme (e.g. Seven). It is somewhat jarring to enter a search term at admin/help in Seven and then see the results show up in a different theme (front-facing theme, e.g. Bartik).

Proposed resolution

The search results should display in the same theme as the search form that was used to submit the search. The search form provider should be able to specify the theme in which the results will display. (Or some other solution.)

Remaining tasks

- Figure out a solution
- Do it.
- Test with User, Node, and Help Topics searches.

User interface changes

We would have the ability to display search results in the administrative theme if the search form is on the admin theme. For example, Help Topics search results would display in the admin theme.

API changes

- Possibly change here in that search form providers might be able to specify a theme for search results to display on.

Data model changes

- None.

Release notes snippet

- TODO

Original report by Amber Himes Matz

Originally reported in the Add search capability to help topics issue, comment #150 by Amber Himes Matz.