When creating a new template, it is all going to be dependent of the Template selected. We would have two options to select from and will mention below.
Change the background color from within the 'Design' portion of your Campaign builder; in doing so it will in turn change the background color for the particular section your content has been placed into (Pre-header, Header, Body, Footer.)
Shown here: https://cl.ly/240q1V333D1p
If you are wanting to place additional text and only wanting to change the background color for the content within the block, you will want to use a 'Boxed Text Content Block'. This will be the only content block that will allow for the background color to be styled for this content block.
Shown here; blue content block: https://cl.ly/0z2Y213E3A32
This article will provide a bit more detail on how to use our Boxed Text Content Block here:
Use Boxed Text Content Blocks: http://eepurl.com/b2Q761