[NTP] Fix voice search dialog background in dark mode

CSS variables cannot be used in pseudo elements.

Bug: 995664
Change-Id: Id607d2b27ba65fd1ca056a89976def2251ea7dea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762577
Commit-Queue: Kristi Park <kristipark@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Auto-Submit: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688702}

chrome/browser/resources/local_ntp/voice.css

@@ -57,7 +57,9 @@
 
 @media (prefers-color-scheme: dark) {
   .overlay-dialog::backdrop {
-    background-color: rgb(var(--dark-mode-dialog-rgb));
+    /* Equivalent to --dark-mode-dialog-rgb. CSS variables cannot be used in
+     * pseudo elements. */
+    background-color: rgb(41, 42, 45);
   }
 }