Align the look&feel of the plugin manager with the intro page

Reuse the same styles of the new intro page in the plugin manager page,
giving a more consistent user experience when switching from one to the
other as part of the out-of-the-box experience of Gerrit Code Review.

Old plugin manager page: https://www.pasteboard.co/2nlqsuEIJxbD.png
New plugin manager page: https://www.pasteboard.co/-Ap5c7i95Yxo.png

Release-Notes: New look&feel for the plugin manager page, aligned with the intro page
Change-Id: I9a4dd3c7111b41ec2561635bf55628dc70783924