You will see the list of groups (communities/enterprises) that you are a member. The groups that you can manage are listed with a link “Manage”. Assuming that you are a community administrator, click on “Manage” link.
Then, click on "Edit Appearance:
Description
You will see the available templates:
default: This template uses 3 columns.
rightbar: In this template, your pages will have 2 columns. Your blocks will be displayed on the collumn on the right.
lefttbar: In this template, your pages will have 2 columns. Your blocks will be displayed on the collumn on the left.
You can change your template clicking on the link “Use this template”