[Redesign] Durable Column touchups

Summary: Ref T8099, slightly cleaner/crisper right gutter, fix icon color, fix date marker.

Test Plan: Review Durable column and scroll behaviour.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D12880