Create your HTML dashboard and place it in the HTML folder you created above. Keep in mind that your HTML page will need to include all of the navigation and toolbars because you’ll be replacing the entire Splunk UI. A good way to get started with an HTML dashboard would be to create a simple XML dashboard and convert it to HTML and then make any further changes you need. You can convert any simple XML dashboard to HTML by opening the dashboard, clicking Edit > Convert to HTML
Next, edit the default.xml file in the nav folder to reference the name of your HTML page (without the extension). To make that page the default, just add default=“true” to the line. You'll want to make sure that a simple XML dashboard with the same name as your HTML dashboard does not exist.