Bug 1576600 - Ensure Learn More link is visible in dark mode. r=andreio

Differential Revision: https://phabricator.services.mozilla.com/D47617

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

browser/themes/shared/customizableui/panelUI.inc.css

@@ -1941,6 +1941,10 @@ panelview[mainview] #PanelUI-whatsNew-content {
   margin-top: 16px;
 }
 
+#protections-popup-message .text-link {
+  color: white;
+}
+
 #protections-popup-message .text-link:hover {
   text-decoration: underline;
   cursor: pointer;