Change background color in texts and Word documents

You can change the background in Messages:

Add backgrounds in Messages on Mac - Apple Support

I've never tried to change the page color in Word. A quick internet search suggested this:

To change the color of the actual paper/page inside your document:

• Go to the Design tab in the top ribbon (or Page Layout in older versions).
• Click Page Color in the Page Background group.
• Choose a theme color, standard color, or click More Colors / Fill Effects for custom shades.