(stable) Add more color/CSS to phui-icon-selector

Summary: Fixes T12205. These got over-ridden at a global scale (correctly) and need to adjust local scopes better. Also make it more bluer.

Test Plan: Go to Edit Dashboard, and pick a new icon for a Dashboard.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12205

Differential Revision: https://secure.phabricator.com/D17312