1. Open the Email Editor in Advance Mode
2. Click on Source in the top left corner
3. Find the text 'templateContainer' which should be towards the top of the file.
To find text easier, use CTR+F (find function) and type in templateContainer to find the text quicker.
4. Add a new line, and enter 'background: #colourcode;
black = background: #000000;
white = background: #ffffff;
5. Click on 'Source' again to go back to the normal view.
Now you can see the text section has changed to a different background colour to the rest of the email.