Go to the 'Email Settings' page from the 'E-mails' tab in your management toolbar.
On this page, check 'Use your own advanced HTML' and click 'Edit HTML template'
On the next page there are a number of options to change colours, fonts and sizes.
You can copy and paste your own HTML code into the box, or write your own code directly. There are no customisation options on this page, so you should only use it if you already know what code you want to use and how to write it.
To include the different sections of text from your e-mail editor, you need to insert comment tags in the place you want your content to show up. More on this can be found on the 'Custom HTML E-mail Template' page itself.
This page allows users to create really great customized e-mails with a lot more scope for changing the layout and the way in which your content is displayed. If you don't fancy writing your own template from scratch, try searching the web for 'Free Newsletter Templates' or similar, and play around with those.
Alternatively:Customize the default e-mail template