update icon color to system color (#28190)

webapp/channels/src/sass/components/_post.scss

@@ -1337,7 +1337,7 @@
             width: 16px;
             height: 20px;
             margin: 0 0 0 4px;
-            color: rgba(61, 60, 64, 0.75);
+            color: rgba(var(--center-channel-color-rgb), 0.75);
             font-size: 16px;
             line-height: 20px;