Bug 1867598 - Fix block button in about:third-party on dark mode. r=desktop-theme-reviewers,Itiel

Differential Revision: https://phabricator.services.mozilla.com/D195117

toolkit/components/aboutthirdparty/content/aboutThirdParty.css

@@ -26,6 +26,8 @@
   background-position: left 8px center;
   background-repeat: no-repeat;
   padding-inline-start: 2em;
+  -moz-context-properties: fill;
+  fill: currentColor;
 }
 
 .button-block:dir(rtl) {