diff --git themes/bartik/css/style.css themes/bartik/css/style.css
index 324c027..0f658cf 100644
--- themes/bartik/css/style.css
+++ themes/bartik/css/style.css
@@ -1360,6 +1360,17 @@ html.js input.throbbing {
   font-size: 0.786em;
 }
 
+/**
+ * Sticky headers.
+ *
+ * Sticky headers use position: fixed to overlap content. However, fieldsets
+ * using position: relative still overlap sticky headers. To make sticky
+ * headers overlap these fieldsets again, a z-index is needed.
+ */
+.sticky-header {
+  z-index: 1;
+}
+
 /* -------------- Other Overrides ------------- */
 
 div.password-suggestions {
diff --git themes/seven/style.css themes/seven/style.css
index a561c08..9adc95d 100644
--- themes/seven/style.css
+++ themes/seven/style.css
@@ -563,6 +563,17 @@ fieldset fieldset fieldset {
 }
 
 /**
+ * Sticky headers.
+ *
+ * Sticky headers use position: fixed to overlap content. However, fieldsets
+ * using position: relative still overlap sticky headers. To make sticky
+ * headers overlap these fieldsets again, a z-index is needed.
+ */
+.sticky-header {
+  z-index: 1;
+}
+
+/**
  * Form elements.
  */
 .form-item {