The default theme is Lucid by Elegant Themes. The theme in a zip format can be downloaded here. All groups are encouraged to select the appropriate themes from elegantthemes.com. You may choose any theme other than Divi. The following is a step by step process for installing the default theme or the one chosen by a given group.
Step 1- Select My Sites from the main bar menu in your group dashboard, choose Network Admin from the menu, and then select Dashboard to get the following screen
Step 2- Select Themes from the dashboard and then choose Add New to get the following screen. Now simply click on Upload Theme (red arrow)
Step 3- Click on Browse to locate your theme in a .zip file (red arrow)
Step 4- Once you locate the theme .zip file, click on Install Now (red arrow)
Step 5- Select network activate (red arrow)
Step 6- Select the dashboard for the group, click on Appearance and then select Themes to display all installed themes in your group website. Hover your mouse over your theme of choice, Lucid here, and then click on the Activate button to complete the installation.