R1:42f30834084e

Add back green drag and drop text area highlight

Summary: Fixes T11884. Better specificity and color.

Test Plan: Drag and drop. Submit.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11884

Differential Revision: https://secure.phabricator.com/D16883