Miles1SW kindly suggested the answer below to create a different background for pages and posts.
The bit I don't understand is the type-page and type-post class selectors.
Would appreciate a link to any documentation on class selectors available in ATO.
Or, would be good to know if such class selectors are only discoverable by working through the code.
Here's Miles' answer:
you perhaps could use the following in ATO>Add HTML/CSS Inserts>CSS Inserts
background: #fff !important;
background: #greenhexcode !important;