The newsletter can be opened in a browser and styled like a normal web page.
You can find the newsletter template in the templates folder of your theme and it is called tpl1.htm.
The important thing to remember is that the placeholder tags stay in place so that the program can replace them and complete the newsletter.
You will see the placeholder tags as tags in brackets example:
[blogname] on line 3
[messbody] on line 95
[messdetails] on line 99 and so forth.
The tags are used by the program to insert the variables, and whe nyou decide to redesign the template its important to take that into consideration.