To import the demo content from the .xml file, you need a designblog-demo-content.xml file which you will get from the downloaded folder (assets > import-data > designblog-demo-content.xml) of Design blog WordPress Theme. If you want the theme to be installed as shown in the demo, you need to import a .xml file into your WordPress dashboard.
Go to Dashboard on the left Sidebar menu, Hover over the Tools and click on Import and scroll the page to the bottom and find WordPress. Click on Install now, after installing you will get Run importer, click on it and choose your designblog-demo-content.xml file, and click on the Upload file and import button. Then assign your author from an existing user (or) a new user and click on Submit and your file is successfully imported. Now, you need to enable Main Menu, click on customize (present in the top bar), a left sidebar will appear. Go to the Menus option, and then select Main Menu scroll to the bottom you will see Menu location select Primary, now your Menu is ready.