Align the primary blue with Gerrit blue-tonal values

By aligning the colors of the plugin-manager and intro screen with
Gerrit header, the user experience will look more fluent and less
fragmented.

Old blue tone: https://www.pasteboard.co/3jGBoYNk7_Eq.png
New blue tone: https://www.pasteboard.co/_EDQs3sdyOEU.png

Release-Notes: Align the intro and plugin-manager headers' colors with Gerrit
Change-Id: Ib47cad3d790536937dc7c8a29e6e2a389f646639
(cherry picked from commit 0aa59e2c834866d8c46a7b2a8a4656e3abf89f82)