Go to the 'Emails' tab in your management toolbar and then click 'Email settings' at the bottom of the left-hand sidebar.
On this page, scroll down to the 'E-mail Template' section, and next to 'Template' check 'Use default template' and click 'Customize Default Template'
In the pop-up window that appears there are a number of options to change colours, fonts and sizes.
Checking 'Custom Header Image' in the 'E-mail Header' section allows you to upload your own image as a banner to be displayed at the top of the e-mail. Bear in mind that this should be the same width as the overall e-mail width you select, also on this page.
'Padding' refers to the blank space between your content and the border.