Issue #2278565 by jstoller, nod_, Bojhan: Clean up module page grouping.

core/themes/seven/style.css

@@ -1037,6 +1037,20 @@ div.admin-options div.form-item {
   border: 0;
   border-top: 1px solid #ccc;
 }
+#system-modules details {
+  border: 0;
+  margin: 0;
+  padding: 0;
+}
+#system-modules summary {
+  border-bottom: 1px solid #ccc;
+}
+#system-modules [open] summary {
+  border-bottom: none;
+}
+#system-modules .details-wrapper {
+  padding: 0 0 0.5em 0;
+}
 #system-modules .fieldset-wrapper {
   padding: 0;
 }