Fix background color for copy link dialog button

In edit mode there is different color for header, therefore
copy link dialog button should use transparent background-color
to be consistent with rest of header.

Google-Bug-Id: b/260344194
Release-Notes: skip
Change-Id: I054646cb938d4f0f7377a6f20313b3bbc56b88af
diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
index c4c37fa..fe2bb4d 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
@@ -923,6 +923,7 @@
         }
         .showCopyLinkDialogButton {
           --gr-button-padding: 0 0 0 var(--spacing-s);
+          --background-color: transparent;
           margin-left: var(--spacing-s);
         }
         #replyBtn {