Default Theme for Blogs Plugin allows site administrators to choose which theme gets activated when a new blog is created. Admin can select the theme from Network Admin->Settings page and this selected theme will be automatically get activated when an existing user create a blog or a new user signs up for a blog and his/her account is activated.
If you are the site Admin, you can access the Settings Menu from Dashboard->NetworkAdmin->Settings
You will see the option to select a theme at the bottom of the settings page as shown below.
