Support both normal and inverted colors for active toolbar tray

Problem/Motivation

If #3097907: Active toolbar tray has weak affordance and fails WCAG color criteria lands, it's going to change the default toolbar styling to invert colors on the active toolbar tray.
If this module doesn't change, it means the #fff icon will disappear against the #fff active background.

Steps to reproduce

1. Install 9.1.x core
2. Apply https://www.drupal.org/files/issues/2020-08-03/3097907-29.patch
3. Install this module
4. Shrink screen width to where the search is a tray, not rendered inline.
5. Click on search tray.
6. Disappearing icon.

Proposed resolution

TBD. Hopefully core will provide a class so we know what style of icon it expects.
E.g. https://www.drupal.org/files/issues/2020-08-03/3097907-32.patch
If you install that instead of 3097907-29.patch and use patch #2 from here, it works.

Remaining tasks

1. Land #3097907: Active toolbar tray has weak affordance and fails WCAG color criteria in core.
2. See how to handle this.
3. Do it.

User interface changes

Hopefully: Icon will always been visible when this toolbar tray is active, regardless of core version.

API changes

N/A