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)

src/main/resources/static/css/intro.css

diff --git a/src/main/resources/static/css/intro.css b/src/main/resources/static/css/intro.css
index 90d0916..ca5a51a 100644
--- a/src/main/resources/static/css/intro.css
+++ b/src/main/resources/static/css/intro.css
@@ -1,6 +1,6 @@
 :root {
-    --primary-blue: #2563eb;
-    --primary-hover: #1d4ed8;
+    --primary-blue: #314972;
+    --primary-hover: #3d5d92;
     --text-dark: #1f2937;
     --text-body: #4b5563;
     --text-light: #9ca3af;