Bug 1741359 - Use auto outline for Windows buttons in dark mode. r=desktop-theme-reviewers,dao

Differential Revision: https://phabricator.services.mozilla.com/D131430

toolkit/themes/windows/global/button.css

@@ -41,6 +41,13 @@ button:focus-visible {
   outline-offset: -3px;
 }
 
+@media (prefers-color-scheme: dark) {
+  button:focus-visible {
+    outline: 1px auto;
+    outline-offset: initial;
+  }
+}
+
 /* .......... default/hover/focused state .......... */
 
 @media (prefers-contrast) and (-moz-windows-compositor) {