--- a/browser/themes/shared/browser.inc.css
+++ b/browser/themes/shared/browser.inc.css
@@ -15,16 +15,33 @@
 :root {
   /* Note: Setting this to 0 (without px) breaks CSS calculations for OSX. */
   --space-above-tabbar: 0px;
   --tabs-navbar-shadow-size: 1px;
   --short-notification-background: #0250BB;
   --short-notification-gradient: #9059FF;
 }
 
+@supports -moz-bool-pref("browser.proton.enabled") {
+  :root {
+    --focus-outline-color: currentColor;
+    --focus-outline: 2px solid var(--focus-outline-color);
+  }
+
+  @media not (prefers-contrast) {
+    :root {
+      --focus-outline-color: #0061E0;
+    }
+
+    :root:-moz-lwtheme-brighttext {
+      --focus-outline-color: #00DDFF;
+    }
+  }
+} /*** END !proton ***/
+
 :root[extradragspace][tabsintitlebar]:not([inFullscreen]) {
   --space-above-tabbar: 8px;
 }
 
 @media (min-resolution: 1.5dppx) {
   :root {
     --tabs-navbar-shadow-size: 0.5px;
   }