fix(cc-dark-theme): minor improvements

- Fix dialog close button color.
- Fix color for error messages when loading threads.
- Fix "duplicate thread" dialog buttons color.

Fixed: twpowertools:193
Change-Id: Ie4048a0faf297b7cad9a75030bf9c63a42914b6d
diff --git a/src/ccDarkTheme/components/_rich-text-editor.scss b/src/ccDarkTheme/components/_rich-text-editor.scss
index 61553f4..2a6d818 100644
--- a/src/ccDarkTheme/components/_rich-text-editor.scss
+++ b/src/ccDarkTheme/components/_rich-text-editor.scss
@@ -29,7 +29,7 @@
 }
 
 material-select-searchbox + material-list material-list-item {
-  color: rgba(255, 255, 255, .87)!important;
+  color: var(--TWPT-icon-color)!important;
 }
 
 ec-attachment .filename {