Add additional indent level

docs/_template/light-dark-theme/styles/master.css

@@ -209,4 +209,19 @@ span.arrow-r{
 .level3 {
     padding: 0 5px;
     font-size: 90%;
+}
+
+.level4 {
+    padding: 0 5px;
+    font-size: 85%;
+}
+
+.level5 {
+    padding: 0 5px;
+    font-size: 80%;
+}
+
+.level6 {
+    padding: 0 5px;
+    font-size: 75%;
 }