Add a dropdown selector for extra context parameter

[simesy]

Hope I'm asking in the right place, I've been looking at the patternlab setup for our drupal integration, but haven't really worked out where this should be done properly.

I want a drop down selector in the main menu (ie next to "Disco" or wherever). I want to populate this with some values.

When the user changes the value, I want to persist the value either as a $_GET in the url, or as a cookie.

The rest I can do easily enough with javascript (eg swap in a stylesheet based on that value) but any other tricks are welcome.