Nesting Levels
This option controls visualization of scopes nesting level. As deeper scope nesting level as darker scope background color.
Feature can be enabled in View→Editor→Nesting Levels for single document or for complete syntax. The color for nesting level is defined in desource_spec.xml, so by default, you can use it in source code files (as PHP or JavaScript), but not in text based syntaxes (as XML or HTML).