Pages allows you to import styles, or copy/paste styles from another open document. Do you believe it is possible to create sufficient variety in external Pages (or MS Word) color schemed documents, that you can import the look you want based on your external document choice? Pages also allows you to size and position images as media placeholders so you can drop an image onto the document and it will resize to fit the placeholder settings.
Pages does not have Themes like Word.
There might be other ways to achieve what you want by swapping master objects and changing colors of Styles. A bit more work but not horrendously more.
You can name color swatches in the Color Picker so you could set up a set of colors there then update styles and objects systematically using those swatches.
Illustrator would be an extremely poor choice for a Newsletter.
Have a look at the new Swift Publisher 3 in the App Store, currently 30% off. They have added some very good tools that have brought it into a much more Pro usable form. Including many key features that Pages lacks.