WebUI Dark Mode: only change bg color of signin confirmation for dark…

… mode

Keep it white in light mode.

Fixed: 1019120
Change-Id: I232652f3784ed4151eff4f1a8a69f5799dc5010b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1895944
Commit-Queue: Dan Beam <dbeam@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711959}

chrome/browser/resources/signin/signin_error/signin_error.html

@@ -13,8 +13,10 @@
     <link rel="import" href="chrome://resources/html/util.html">
     <custom-style>
       <style include="signin-dialog-shared">
-        html {
-          background: var(--md-background-color);
+        @media (prefers-color-scheme: dark) {
+          html {
+            background: var(--md-background-color);
+          }
         }
 
         .details {