WebUI Dark Mode: update #deep-links on chrome://flags for dark mode

I previously changed the --primary-color to a light gray for dark mode,
but didn't update the highlighted text when one links to a flag via

chrome://flags#deep-link

Which highlights the background in yellow (which looks bad and has
terrible contrast with light gray in dark mode).

Just force dark mode into the same light mode UI for this. Screenshot on
bug.

Fixed: 999031
Change-Id: I34b3933a08d96523eeac9bdd1a80b91e402b96bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1885344
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@{#710189}

components/flags_ui/resources/flags.css

@@ -316,7 +316,9 @@ button:not(.primary):hover {
 
 .match,
 .referenced h3 {
+  /* This UI is intentionally the same in light and dark mode. */
   background: yellow;
+  color: var(--google-grey-900);
 }
 
 .experiment-actions {