Chrome OS First run: Update font colors

Bug: 890457
Change-Id: I73bcf6af092e99d77fc1c28322737408acbc9446
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1871672Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708499}

chrome/browser/resources/chromeos/first_run/step.css

@@ -9,11 +9,12 @@
 }
 
 .step h1 {
-  color: #333;
+  color: rgb(32, 33, 36); /* #202124 */
   font-weight: normal;
 }
 
 .step p {
+  color: rgb(95, 99, 104); /* #5F6368 */
   line-height: 1.5;
 }