Bug 1489495 - Fix synced tabs sidebar instructions text color with WebExtension themes. r=dao

Differential Revision: https://phabricator.services.mozilla.com/D5257

--HG--
extra : moz-landing-system : lando

browser/themes/shared/syncedtabs/sidebar.inc.css

@@ -196,7 +196,6 @@ body {
 .sync-state > p {
   padding-inline-end: 10px;
   padding-inline-start: 10px;
-  color: #888;
 }
 
 .text-link {
@@ -255,6 +254,11 @@ body {
   margin: 0 auto;
 }
 
+body[lwt-sidebar] .deck .instructions {
+  color: inherit;
+  opacity: .6;
+}
+
 .deck .button {
   display: block;
   background-color: #0060df;