You cannot modify the default template, however copying an existing newsletter works well and often saves much re-entry.
Unsubscribe links will be taken care of for you in the footer of your email, however you can include the following fields in your email content
If you want to use background images and more complex items, then more technical users may refer to the email stylesheet in the custom templates design area.
You can also use additional fields inside square tags 
And for the sake of completeness and brevity, you can also use any of these, in both the subject and email body. And tags are case insensitive.
mbr_terms_agreed, mbr_debtcollect, mbr_balance,