Add a dark/light mode selector on the survey results sites for accessibility

Some people (myself included) find dark mode difficult to work with (I get visual aftereffects for about 20 minutes after reading web pages in dark mode for anything more than a brief scan). The format of the page doesn't trigger browsers that offer an accessible reading mode to make that visible (they're usually suppressed on navigation pages and only shown on article pages), so the survey is hard for a percentage of the audience to read.

It would be great to have a selector on the page to switch from dark to light mode, perhaps alongside the list of multiple languages to switch the sites into. thanks!

No files or tests are named. Start by locating the shared survey-results navigation and the existing language selector, then determine where a light/dark mode control belongs across the sites. Done means users can switch modes and the result is readable for people who find dark mode difficult.

Written by the indexing model from the issue text.