Fix dark highlight in diff for user suggested edits

To fix inline changes color in diff for user suggested edits, setting
var --dark-add-highlight-color was moved from whole gr-comment-thread
to just gr-diff#diff inside of gr-comment-thread. So it doesn't apply
on diff inside of gr-user-suggestion-fix.

Screenshot: https://imgur.com/a/VhZmf7I
Release-Notes: skip
Google-Bug-Id: b/277102018
Change-Id: Id7796cec174c8385087a5ed603d9af59cb04c3ac
diff --git a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
index 30576c8..24777fb 100644
--- a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
+++ b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
@@ -315,6 +315,8 @@
           font-size: var(--font-size-normal);
           font-weight: var(--font-weight-normal);
           line-height: var(--line-height-normal);
+        }
+        gr-diff#diff {
           /* Explicitly set the background color of the diff. We
            * cannot use the diff content type ab because of the skip chunk preceding
            * it, diff processor assumes the chunk of type skip/ab can be collapsed