[signin] Update reauth dialog style to match the mocks more closely

This CL applies 3 minor changes to the reauth dialog style:
- Left-align the title
- Use the grey color for the body text
- Reduce the margin the title and the body text

Bug: 1107522
Change-Id: I6d37b4fc968a89dac96e94157c93a427de7ac924
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2307949
Auto-Submit: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Esmael Elmoslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790195}

chrome/browser/resources/signin/signin_reauth/signin_reauth_app.html

@@ -45,12 +45,12 @@
 
   h1 {
     font-weight: normal;
-    margin-bottom: 32px;
+    margin-bottom: 8px;
     padding: 0 40px;
-    text-align: center;
   }
 
   .message-container {
+    color: var(--cr-secondary-text-color);
     line-height: 20px;
     margin-bottom: 16px;
     padding: 0 40px;