Bug 1521837 - Fix margin-inline-end on about:addons options button r=dao

Differential Revision: https://phabricator.services.mozilla.com/D17592

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

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

@@ -254,6 +254,7 @@ button.warning {
   -moz-context-properties: fill;
   fill: currentColor;
   min-width: auto;
+  margin-inline-end: 0;
 }
 
 #header-utils-btn > .button-box > hbox > .button-icon {