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You can't modify or edit the default template, you need to clone the default template set first. Go to System Admin -> Templates and click on the "clone" link. You can call this clone anything you wish, or name it similar to the theme you are using. Then go to system admin/preferences/general settings and set your default "Template" to your named cloned template.
Then go back to system admin-->templates, your default clone will have a tick to the right of it.