Omnibasis offers a number of default themes for your dashboard layout. You can change your default site layout by going to 'omniSite \ Site layout for Dashboard', 

Note: you can separately manage site layout and dashboard layout. Learn how to manage Site Layout.

Navigate to  'omniSite \ Site layout for Dashboard' to change a layout.

Choose a new layout for dashboard navigation and click Save as application default

Your dashboard layout is your default layout when user visits your application on Omnibasis and login. By default, users can change user their preferred layout under 'My Account \ Site layout for dashboard'. You can disable that by changing app permissions under 'omniCustomers \ Roles' for the default User role.