fix(cc-dark-theme): improve dark theme

- Fixed canned response row hover background color.
- Fixed background color of the nested thread view "{num} More comments"
  button.

Fixed: twpowertools:178
Change-Id: I758a997a69319fc41dacd4a7363fe94db0b81dc4
diff --git a/src/ccDarkTheme/vendors/google/_thread.scss b/src/ccDarkTheme/vendors/google/_thread.scss
index e29c25a..86b370b 100644
--- a/src/ccDarkTheme/vendors/google/_thread.scss
+++ b/src/ccDarkTheme/vendors/google/_thread.scss
@@ -209,6 +209,14 @@
   color: var(--TWPT-interop-secondary-text);
 }
 
+.scTailwindThreadMessagegapmore-nested-replies {
+  background: #3c4043;
+}
+
+.scTailwindThreadMessagegapmore-nested-replies-divider {
+  background: #27282b;
+}
+
 .scTailwindThreadPesignupdialogclose-button-row button {
   color: #fff;
 }