Bug 1401917 - Let tab strip scroll buttons use --toolbarbutton-icon-fill-opacity. r=nhnt11

MozReview-Commit-ID: 3z9z1Yw4SNc

--HG--
extra : rebase_source : ad6cb436eb8f2570f31414317913388c55ef799e

browser/themes/shared/tabs.inc.css

@@ -565,7 +565,7 @@ tabbrowser {
   list-style-image: url(chrome://browser/skin/arrow-left.svg);
   -moz-context-properties: fill, fill-opacity;
   fill: currentColor;
-  fill-opacity: .8;
+  fill-opacity: var(--toolbarbutton-icon-fill-opacity);
 }
 
 .tabbrowser-arrowscrollbox > .scrollbutton-up:-moz-locale-dir(rtl),