Bug 1526269 - Mirroring Firefox's default and lightweight theme icons on about:addons. r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D21620

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

toolkit/themes/shared/extensions/extensions.inc.css

@@ -474,6 +474,10 @@ button.warning {
   -moz-box-align: center;
 }
 
+.addon-view[type="theme"] .icon:-moz-locale-dir(rtl) {
+  transform: scaleX(-1);
+}
+
 .content-container {
   -moz-box-align: center;
 }