For all Joomla 3+ templates built using the Zen Grid Framework v4 (any theme after October 2014) please refer to the Zen Grid Framework v4 documentation.
The diagnostics panel helps you to keep track of important information regarding your current template configuration.
Inside Diagnostics panel you get warning messages. Like on image you can see warning about hidden content area on homepage. This should be always first place for debugging your site.
Next step for debugging your site issues is enabling Template Debugging. Please understand that template debugging is different to Joomla debugging!
After you enable debugging you can view all template positions, all loaded css and all loaded js files including their path. This is helpful in case something is not working right...usually you will find that your other extension is also loading jQuery library so you will need to disable this extra jQuery.
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Legacy Mode is an option that helps to ease the challenge of backwards compatibility with templates that were first built using v1.1 of the framework. This option maintains the same markup that was used in v1.1 of the framework but as a result does not allow the display of debug information or social icons on the front end of your site.
The layout file used to render the legacy layout can be found in the zengridframework/legacy/layout/legacy.php file.