diff -u b/core/themes/bartik/css/style.css b/core/themes/bartik/css/style.css
--- b/core/themes/bartik/css/style.css
+++ b/core/themes/bartik/css/style.css
@@ -1304,6 +1304,21 @@
   border: 1px solid #bbb;
   border-bottom: 1px solid #fff;
 }
+@media screen and (max-width: 37.5em) { /* 600px */
+  .tabs ul.primary {
+    border-bottom: 1px solid #bbb;
+  }
+  .tabs ul.primary li {
+    display: block;
+    margin: 0;
+  }
+  .tabs ul.primary li a {
+    padding: 5px 10px;
+  }
+  .tabs ul.primary li.active a {
+    border-bottom: none;
+  }
+}
 .tabs--secondary {
   border-bottom: none;
   margin: 5px;
diff -u b/core/themes/seven/css/components/tabs.css b/core/themes/seven/css/components/tabs.css
--- b/core/themes/seven/css/components/tabs.css
+++ b/core/themes/seven/css/components/tabs.css
@@ -19,8 +19,8 @@
 }
 
 /* Span the full width of the viewport */
-#branding .is-horizontal .tabs:before,
-#branding .is-collapse-enabled .tabs:before {
+.content-header .is-horizontal .tabs:before,
+.content-header .is-collapse-enabled .tabs:before {
   left: -2.5em;
   right: -2.5em;
 }