Fix invalid CSS border-left-color value for .phabricator-action-view-submenu .caret-right

Summary:
The W3C CSS validator throws Value Error : border-left-color Too many values or values are not recognized : 71,87,120.
The value lacks rgb() wrapping. See other lines in the same CSS file.

Test Plan: Paste celerity processed phui-action-list.css into https://jigsaw.w3.org/css-validator/; look at the browser console while editing a task.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25925