Improve color use in "[+++- ]" element for colorblind users

Summary:
Ref T13127. Users with red/green colorblindness may have difficulty using this element in its current incarnation.

We could give it different behavior if the "Accessibility" option is set for red/green colorblind users, but try a one-size-fits-all approach since the red/green aren't wholly clear anwyay.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13127

Differential Revision: https://secure.phabricator.com/D19385