WebUI Dark Mode: fix "create user" input text color

Screenshot: https://imgur.com/a/wLZIEi0

R=hcarmona@chromium.org
BUG=935238

Change-Id: I080af135e2754b2cc723d276eebbe465f6e2a958
Reviewed-on: https://chromium-review.googlesource.com/c/1488033
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635856}

chrome/browser/resources/md_user_manager/create_profile.html

@@ -81,6 +81,7 @@
       }
 
       #nameInput {
+        --cr-input-color: var(--cr-primary-text-color);
         --cr-input-input: {
           border-bottom: 1px solid var(--cr-secondary-text-color);
         }